refactor(backend): add validation patterns, query validators, and test builders

- Add centralized ValidationPatterns with XSS protection and name regex
- Add query validators (GetClassmates, GetStudentById, GetStudentsPaged, GetAvailableSubjects)
- Add ClassmateInfo read model for optimized query projections
- Add test builders (Student, Subject, Professor, Enrollment) for cleaner tests
- Improve repository interfaces with XML documentation
- Refactor EnrollmentRepository for better performance
- Update EnrollmentDomainService with additional validation helpers

src/backend/Adapters/Driven/Persistence/Repositories/EnrollmentRepository.cs

@@ -3,8 +3,12 @@ namespace Adapters.Driven.Persistence.Repositories;
 using Adapters.Driven.Persistence.Context;
 using Domain.Entities;
 using Domain.Ports.Repositories;
+using Domain.ReadModels;
 using Microsoft.EntityFrameworkCore;
 
+/// <summary>
+/// EF Core implementation of <see cref="IEnrollmentRepository"/>.
+/// </summary>
 public class EnrollmentRepository(AppDbContext context) : IEnrollmentRepository
 {
     public async Task<Enrollment?> GetByIdAsync(int id, CancellationToken ct = default) =>
@@ -29,45 +33,32 @@ public class EnrollmentRepository(AppDbContext context) : IEnrollmentRepository
             .AsNoTracking()
             .ToListAsync(ct);
 
-    public async Task<IReadOnlyList<Student>> GetClassmatesAsync(int studentId, int subjectId, CancellationToken ct = default) =>
+    public async Task<IReadOnlyList<ClassmateInfo>> GetClassmatesAsync(int studentId, int subjectId, CancellationToken ct = default) =>
         await context.Enrollments
             .Where(e => e.SubjectId == subjectId && e.StudentId != studentId)
-            .Select(e => e.Student)
+            .Select(e => new ClassmateInfo(e.Student.Id, e.Student.Name))
-            .AsNoTracking()
             .ToListAsync(ct);
 
-    public async Task<IReadOnlyDictionary<int, IReadOnlyList<Student>>> GetClassmatesBatchAsync(
+    public async Task<IReadOnlyDictionary<int, IReadOnlyList<ClassmateInfo>>> GetClassmatesBatchAsync(
         int studentId, IEnumerable<int> subjectIds, CancellationToken ct = default)
     {
         var subjectIdList = subjectIds.ToList();
         if (subjectIdList.Count == 0)
-            return new Dictionary<int, IReadOnlyList<Student>>();
+            return new Dictionary<int, IReadOnlyList<ClassmateInfo>>();
 
-        // Single query to get all classmates for all subjects
+        // Single query to get all classmates for all subjects - projects directly to ClassmateInfo
+        // Note: AsNoTracking is not needed for projections (Select) as they're not tracked by default
         var enrollments = await context.Enrollments
             .Where(e => subjectIdList.Contains(e.SubjectId) && e.StudentId != studentId)
-            .Select(e => new { e.SubjectId, e.Student.Id, e.Student.Name })
+            .Select(e => new { e.SubjectId, Classmate = new ClassmateInfo(e.Student.Id, e.Student.Name) })
-            .AsNoTracking()
             .ToListAsync(ct);
 
-        // Group by SubjectId and project to Student (minimal data needed)
+        // Group by SubjectId
         return enrollments
             .GroupBy(e => e.SubjectId)
             .ToDictionary(
                 g => g.Key,
-                g => (IReadOnlyList<Student>)g
+                g => (IReadOnlyList<ClassmateInfo>)g.Select(e => e.Classmate).ToList());
-                    .Select(e => CreateMinimalStudent(e.Id, e.Name))
-                    .ToList());
-    }
-
-    private static Student CreateMinimalStudent(int id, string name)
-    {
-        // Use reflection to set Id since it's private set
-        var student = (Student)System.Runtime.CompilerServices.RuntimeHelpers
-            .GetUninitializedObject(typeof(Student));
-        typeof(Student).GetProperty("Id")!.SetValue(student, id);
-        typeof(Student).GetProperty("Name")!.SetValue(student, name);
-        return student;
     }
 
     public void Add(Enrollment enrollment) => context.Enrollments.Add(enrollment);

src/backend/Application/Common/ValidationPatterns.cs

@@ -0,0 +1,69 @@
+namespace Application.Common;
+
+using System.Text.RegularExpressions;
+
+/// <summary>
+/// Shared validation patterns and constants for input validation.
+/// Used across validators to ensure consistency and DRY principle.
+/// </summary>
+public static partial class ValidationPatterns
+{
+    #region Name Validation
+
+    /// <summary>
+    /// Minimum length for student/person names.
+    /// </summary>
+    public const int MinNameLength = 3;
+
+    /// <summary>
+    /// Maximum length for student/person names.
+    /// </summary>
+    public const int MaxNameLength = 100;
+
+    /// <summary>
+    /// Pattern for valid name characters.
+    /// Allows: Unicode letters, spaces, hyphens, apostrophes, and periods.
+    /// Examples: "Juan Pérez", "María O'Brien", "Jean-Pierre"
+    /// </summary>
+    [GeneratedRegex(@"^[\p{L}\s\-'\.]+$", RegexOptions.Compiled)]
+    public static partial Regex SafeNamePattern();
+
+    #endregion
+
+    #region Email Validation
+
+    /// <summary>
+    /// Maximum length for email addresses (RFC 5321).
+    /// </summary>
+    public const int MaxEmailLength = 254;
+
+    #endregion
+
+    #region Security Patterns
+
+    /// <summary>
+    /// Pattern to detect potentially dangerous content (XSS, script injection).
+    /// Matches: HTML tags, javascript: protocol, event handlers (onclick, onload, etc.)
+    /// </summary>
+    [GeneratedRegex(@"<[^>]*>|javascript:|on\w+=", RegexOptions.IgnoreCase | RegexOptions.Compiled)]
+    public static partial Regex DangerousContentPattern();
+
+    /// <summary>
+    /// Checks if a string contains potentially dangerous content.
+    /// </summary>
+    /// <param name="value">The string to check.</param>
+    /// <returns>True if the content is safe (no dangerous patterns found).</returns>
+    public static bool IsSafeContent(string? value) =>
+        string.IsNullOrEmpty(value) || !DangerousContentPattern().IsMatch(value);
+
+    #endregion
+
+    #region ID Validation
+
+    /// <summary>
+    /// Error message for invalid entity ID.
+    /// </summary>
+    public const string InvalidIdMessage = "ID must be greater than 0";
+
+    #endregion
+}

src/backend/Application/Enrollments/Commands/EnrollStudentValidator.cs

@@ -1,24 +1,33 @@
 namespace Application.Enrollments.Commands;
 
+using Application.Common;
 using FluentValidation;
 
+/// <summary>
+/// Validator for <see cref="EnrollStudentCommand"/>.
+/// Validates that student and subject IDs are valid.
+/// </summary>
 public class EnrollStudentValidator : AbstractValidator<EnrollStudentCommand>
 {
     public EnrollStudentValidator()
     {
         RuleFor(x => x.StudentId)
-            .GreaterThan(0).WithMessage("Invalid student ID");
+            .GreaterThan(0).WithMessage(ValidationPatterns.InvalidIdMessage);
 
         RuleFor(x => x.SubjectId)
-            .GreaterThan(0).WithMessage("Invalid subject ID");
+            .GreaterThan(0).WithMessage(ValidationPatterns.InvalidIdMessage);
     }
 }
 
+/// <summary>
+/// Validator for <see cref="UnenrollStudentCommand"/>.
+/// Validates that enrollment ID is valid.
+/// </summary>
 public class UnenrollStudentValidator : AbstractValidator<UnenrollStudentCommand>
 {
     public UnenrollStudentValidator()
     {
         RuleFor(x => x.EnrollmentId)
-            .GreaterThan(0).WithMessage("Invalid enrollment ID");
+            .GreaterThan(0).WithMessage(ValidationPatterns.InvalidIdMessage);
     }
 }

src/backend/Application/Enrollments/Queries/GetClassmatesQuery.cs

@@ -5,8 +5,16 @@ using Domain.Exceptions;
 using Domain.Ports.Repositories;
 using MediatR;
 
+/// <summary>
+/// Query to retrieve classmates for all subjects a student is enrolled in.
+/// </summary>
+/// <param name="StudentId">The student's unique identifier.</param>
 public record GetClassmatesQuery(int StudentId) : IRequest<IReadOnlyList<ClassmatesBySubjectDto>>;
 
+/// <summary>
+/// Handler for <see cref="GetClassmatesQuery"/>.
+/// Uses batch query to avoid N+1 problem.
+/// </summary>
 public class GetClassmatesHandler(
     IStudentRepository studentRepository,
     IEnrollmentRepository enrollmentRepository)
@@ -31,7 +39,7 @@ public class GetClassmatesHandler(
             enrollment.SubjectId,
             enrollment.Subject?.Name ?? "",
             classmatesBySubject.TryGetValue(enrollment.SubjectId, out var classmates)
-                ? classmates.Select(c => new ClassmateDto(c.Id, c.Name)).ToList()
+                ? classmates.Select(c => new ClassmateDto(c.StudentId, c.Name)).ToList()
                 : []
         )).ToList();
     }

src/backend/Application/Enrollments/Queries/GetClassmatesValidator.cs

@@ -0,0 +1,17 @@
+namespace Application.Enrollments.Queries;
+
+using Application.Common;
+using FluentValidation;
+
+/// <summary>
+/// Validator for <see cref="GetClassmatesQuery"/>.
+/// Ensures the student ID is valid before querying classmates.
+/// </summary>
+public class GetClassmatesValidator : AbstractValidator<GetClassmatesQuery>
+{
+    public GetClassmatesValidator()
+    {
+        RuleFor(x => x.StudentId)
+            .GreaterThan(0).WithMessage(ValidationPatterns.InvalidIdMessage);
+    }
+}

src/backend/Application/Students/Commands/CreateStudentValidator.cs

@@ -1,70 +1,72 @@
 namespace Application.Students.Commands;
 
-using System.Text.RegularExpressions;
+using Application.Common;
 using Domain.Ports.Repositories;
 using FluentValidation;
 
-public partial class CreateStudentValidator : AbstractValidator<CreateStudentCommand>
+/// <summary>
+/// Validator for <see cref="CreateStudentCommand"/>.
+/// Validates name format, email uniqueness, and prevents XSS attacks.
+/// </summary>
+public class CreateStudentValidator : AbstractValidator<CreateStudentCommand>
 {
-    [GeneratedRegex(@"^[\p{L}\s\-'\.]+$", RegexOptions.Compiled)]
-    private static partial Regex SafeNamePattern();
-
-    [GeneratedRegex(@"<[^>]*>|javascript:|on\w+=", RegexOptions.IgnoreCase | RegexOptions.Compiled)]
-    private static partial Regex DangerousPattern();
-
     public CreateStudentValidator(IStudentRepository studentRepository)
     {
         RuleFor(x => x.Name)
             .NotEmpty().WithMessage("Name is required")
-            .MinimumLength(3).WithMessage("Name must be at least 3 characters")
+            .MinimumLength(ValidationPatterns.MinNameLength)
-            .MaximumLength(100).WithMessage("Name must not exceed 100 characters")
+                .WithMessage($"Name must be at least {ValidationPatterns.MinNameLength} characters")
-            .Matches(SafeNamePattern()).WithMessage("Name contains invalid characters")
+            .MaximumLength(ValidationPatterns.MaxNameLength)
-            .Must(NotContainDangerousContent).WithMessage("Name contains prohibited content");
+                .WithMessage($"Name must not exceed {ValidationPatterns.MaxNameLength} characters")
+            .Matches(ValidationPatterns.SafeNamePattern())
+                .WithMessage("Name contains invalid characters")
+            .Must(ValidationPatterns.IsSafeContent)
+                .WithMessage("Name contains prohibited content");
 
         RuleFor(x => x.Email)
             .NotEmpty().WithMessage("Email is required")
-            .MaximumLength(254).WithMessage("Email must not exceed 254 characters")
+            .MaximumLength(ValidationPatterns.MaxEmailLength)
+                .WithMessage($"Email must not exceed {ValidationPatterns.MaxEmailLength} characters")
             .EmailAddress().WithMessage("Invalid email format")
-            .Must(NotContainDangerousContent).WithMessage("Email contains prohibited content")
+            .Must(ValidationPatterns.IsSafeContent)
+                .WithMessage("Email contains prohibited content")
             .MustAsync(async (email, ct) =>
                 !await studentRepository.EmailExistsAsync(email, null, ct))
             .WithMessage("Email already exists");
     }
-
-    private static bool NotContainDangerousContent(string? value) =>
-        string.IsNullOrEmpty(value) || !DangerousPattern().IsMatch(value);
 }
 
-public partial class UpdateStudentValidator : AbstractValidator<UpdateStudentCommand>
+/// <summary>
+/// Validator for <see cref="UpdateStudentCommand"/>.
+/// Validates name format, email uniqueness (excluding current student), and prevents XSS attacks.
+/// </summary>
+public class UpdateStudentValidator : AbstractValidator<UpdateStudentCommand>
 {
-    [GeneratedRegex(@"^[\p{L}\s\-'\.]+$", RegexOptions.Compiled)]
-    private static partial Regex SafeNamePattern();
-
-    [GeneratedRegex(@"<[^>]*>|javascript:|on\w+=", RegexOptions.IgnoreCase | RegexOptions.Compiled)]
-    private static partial Regex DangerousPattern();
-
     public UpdateStudentValidator(IStudentRepository studentRepository)
     {
         RuleFor(x => x.Id)
-            .GreaterThan(0).WithMessage("Invalid student ID");
+            .GreaterThan(0).WithMessage(ValidationPatterns.InvalidIdMessage);
 
         RuleFor(x => x.Name)
             .NotEmpty().WithMessage("Name is required")
-            .MinimumLength(3).WithMessage("Name must be at least 3 characters")
+            .MinimumLength(ValidationPatterns.MinNameLength)
-            .MaximumLength(100).WithMessage("Name must not exceed 100 characters")
+                .WithMessage($"Name must be at least {ValidationPatterns.MinNameLength} characters")
-            .Matches(SafeNamePattern()).WithMessage("Name contains invalid characters")
+            .MaximumLength(ValidationPatterns.MaxNameLength)
-            .Must(NotContainDangerousContent).WithMessage("Name contains prohibited content");
+                .WithMessage($"Name must not exceed {ValidationPatterns.MaxNameLength} characters")
+            .Matches(ValidationPatterns.SafeNamePattern())
+                .WithMessage("Name contains invalid characters")
+            .Must(ValidationPatterns.IsSafeContent)
+                .WithMessage("Name contains prohibited content");
 
         RuleFor(x => x.Email)
             .NotEmpty().WithMessage("Email is required")
-            .MaximumLength(254).WithMessage("Email must not exceed 254 characters")
+            .MaximumLength(ValidationPatterns.MaxEmailLength)
+                .WithMessage($"Email must not exceed {ValidationPatterns.MaxEmailLength} characters")
             .EmailAddress().WithMessage("Invalid email format")
-            .Must(NotContainDangerousContent).WithMessage("Email contains prohibited content")
+            .Must(ValidationPatterns.IsSafeContent)
+                .WithMessage("Email contains prohibited content")
             .MustAsync(async (command, email, ct) =>
                 !await studentRepository.EmailExistsAsync(email, command.Id, ct))
             .WithMessage("Email already exists");
     }
-
-    private static bool NotContainDangerousContent(string? value) =>
-        string.IsNullOrEmpty(value) || !DangerousPattern().IsMatch(value);
 }

src/backend/Application/Students/Queries/GetStudentByIdValidator.cs

@@ -0,0 +1,17 @@
+namespace Application.Students.Queries;
+
+using Application.Common;
+using FluentValidation;
+
+/// <summary>
+/// Validator for <see cref="GetStudentByIdQuery"/>.
+/// Ensures the student ID is valid before querying.
+/// </summary>
+public class GetStudentByIdValidator : AbstractValidator<GetStudentByIdQuery>
+{
+    public GetStudentByIdValidator()
+    {
+        RuleFor(x => x.Id)
+            .GreaterThan(0).WithMessage(ValidationPatterns.InvalidIdMessage);
+    }
+}

src/backend/Application/Students/Queries/GetStudentsPagedValidator.cs

@@ -0,0 +1,22 @@
+namespace Application.Students.Queries;
+
+using Application.Common;
+using FluentValidation;
+
+/// <summary>
+/// Validator for <see cref="GetStudentsPagedQuery"/>.
+/// Validates pagination parameters.
+/// </summary>
+public class GetStudentsPagedValidator : AbstractValidator<GetStudentsPagedQuery>
+{
+    public GetStudentsPagedValidator()
+    {
+        RuleFor(x => x.PageSize)
+            .GreaterThan(0).WithMessage("Page size must be greater than 0")
+            .LessThanOrEqualTo(50).WithMessage("Page size must not exceed 50");
+
+        RuleFor(x => x.AfterCursor)
+            .GreaterThan(0).WithMessage(ValidationPatterns.InvalidIdMessage)
+            .When(x => x.AfterCursor.HasValue);
+    }
+}

src/backend/Application/Subjects/Queries/GetAvailableSubjectsQuery.cs

@@ -1,16 +1,24 @@
 namespace Application.Subjects.Queries;
 
 using Application.Subjects.DTOs;
-using Domain.Entities;
 using Domain.Exceptions;
 using Domain.Ports.Repositories;
+using Domain.Services;
 using MediatR;
 
+/// <summary>
+/// Query to get all subjects with availability status for a specific student.
+/// </summary>
 public record GetAvailableSubjectsQuery(int StudentId) : IRequest<IReadOnlyList<AvailableSubjectDto>>;
 
+/// <summary>
+/// Handler for <see cref="GetAvailableSubjectsQuery"/>.
+/// Uses domain service for availability logic.
+/// </summary>
 public class GetAvailableSubjectsHandler(
     IStudentRepository studentRepository,
-    ISubjectRepository subjectRepository)
+    ISubjectRepository subjectRepository,
+    EnrollmentDomainService enrollmentDomainService)
     : IRequestHandler<GetAvailableSubjectsQuery, IReadOnlyList<AvailableSubjectDto>>
 {
     public async Task<IReadOnlyList<AvailableSubjectDto>> Handle(
@@ -22,41 +30,17 @@ public class GetAvailableSubjectsHandler(
 
         var allSubjects = await subjectRepository.GetAllWithProfessorsAsync(ct);
 
-        var enrolledSubjectIds = student.Enrollments.Select(e => e.SubjectId).ToHashSet();
+        return allSubjects.Select(subject =>
-        var enrolledProfessorIds = student.Enrollments
-            .Select(e => e.Subject?.ProfessorId ?? 0)
-            .Where(id => id > 0)
-            .ToHashSet();
-
-        return allSubjects.Select(s =>
         {
-            var (isAvailable, reason) = GetAvailability(s, student, enrolledSubjectIds, enrolledProfessorIds);
+            var (isAvailable, reason) = enrollmentDomainService.CheckSubjectAvailability(student, subject);
             return new AvailableSubjectDto(
-                s.Id,
+                subject.Id,
-                s.Name,
+                subject.Name,
-                s.Credits,
+                subject.Credits,
-                s.ProfessorId,
+                subject.ProfessorId,
-                s.Professor?.Name ?? "",
+                subject.Professor?.Name ?? "",
                 isAvailable,
                 reason);
         }).ToList();
     }
-
-    private static (bool IsAvailable, string? Reason) GetAvailability(
-        Subject subject,
-        Student student,
-        HashSet<int> enrolledSubjectIds,
-        HashSet<int> enrolledProfessorIds)
-    {
-        if (enrolledSubjectIds.Contains(subject.Id))
-            return (false, "Already enrolled");
-
-        if (!student.CanEnroll())
-            return (false, "Maximum 3 subjects reached");
-
-        if (enrolledProfessorIds.Contains(subject.ProfessorId))
-            return (false, "Already have a subject with this professor");
-
-        return (true, null);
-    }
 }

src/backend/Application/Subjects/Queries/GetAvailableSubjectsValidator.cs

@@ -0,0 +1,17 @@
+namespace Application.Subjects.Queries;
+
+using Application.Common;
+using FluentValidation;
+
+/// <summary>
+/// Validator for <see cref="GetAvailableSubjectsQuery"/>.
+/// Ensures the student ID is valid before querying available subjects.
+/// </summary>
+public class GetAvailableSubjectsValidator : AbstractValidator<GetAvailableSubjectsQuery>
+{
+    public GetAvailableSubjectsValidator()
+    {
+        RuleFor(x => x.StudentId)
+            .GreaterThan(0).WithMessage(ValidationPatterns.InvalidIdMessage);
+    }
+}

src/backend/Domain/Exceptions/DomainException.cs

@@ -1,7 +1,14 @@
 namespace Domain.Exceptions;
 
+/// <summary>
+/// Base class for all domain exceptions.
+/// Provides a machine-readable code for client error handling.
+/// </summary>
 public abstract class DomainException : Exception
 {
+    /// <summary>
+    /// Machine-readable error code for client handling.
+    /// </summary>
     public string Code { get; }
 
     protected DomainException(string code, string message) : base(message)
@@ -10,38 +17,75 @@ public abstract class DomainException : Exception
     }
 }
 
+/// <summary>
+/// Thrown when a student attempts to enroll in more than 3 subjects.
+/// </summary>
 public class MaxEnrollmentsExceededException : DomainException
 {
     public MaxEnrollmentsExceededException(int studentId)
         : base("MAX_ENROLLMENTS", $"Student {studentId} cannot enroll in more than 3 subjects") { }
 }
 
+/// <summary>
+/// Thrown when a student attempts to enroll in a subject with a professor they already have.
+/// </summary>
 public class SameProfessorConstraintException : DomainException
 {
     public SameProfessorConstraintException(int studentId, int professorId)
         : base("SAME_PROFESSOR", $"Student {studentId} already has a subject with professor {professorId}") { }
 }
 
+/// <summary>
+/// Thrown when a requested student is not found.
+/// </summary>
 public class StudentNotFoundException : DomainException
 {
     public StudentNotFoundException(int studentId)
         : base("STUDENT_NOT_FOUND", $"Student {studentId} was not found") { }
 }
 
+/// <summary>
+/// Thrown when a requested subject is not found.
+/// </summary>
 public class SubjectNotFoundException : DomainException
 {
     public SubjectNotFoundException(int subjectId)
         : base("SUBJECT_NOT_FOUND", $"Subject {subjectId} was not found") { }
 }
 
+/// <summary>
+/// Thrown when a requested enrollment is not found.
+/// </summary>
 public class EnrollmentNotFoundException : DomainException
 {
     public EnrollmentNotFoundException(int enrollmentId)
         : base("ENROLLMENT_NOT_FOUND", $"Enrollment {enrollmentId} was not found") { }
 }
 
+/// <summary>
+/// Thrown when a student attempts to enroll in a subject they're already enrolled in.
+/// </summary>
 public class DuplicateEnrollmentException : DomainException
 {
     public DuplicateEnrollmentException(int studentId, int subjectId)
         : base("DUPLICATE_ENROLLMENT", $"Student {studentId} is already enrolled in subject {subjectId}") { }
 }
+
+/// <summary>
+/// Thrown when attempting to assign more than 2 subjects to a professor.
+/// Business rule: Each professor teaches exactly 2 subjects.
+/// </summary>
+public class ProfessorMaxSubjectsExceededException : DomainException
+{
+    public ProfessorMaxSubjectsExceededException(int professorId)
+        : base("PROFESSOR_MAX_SUBJECTS", $"Professor {professorId} already has the maximum of 2 subjects") { }
+}
+
+/// <summary>
+/// Thrown when a requested professor is not found.
+/// </summary>
+public class ProfessorNotFoundException : DomainException
+{
+    public ProfessorNotFoundException(int professorId)
+        : base("PROFESSOR_NOT_FOUND", $"Professor {professorId} was not found") { }
+}

src/backend/Domain/Ports/Repositories/IEnrollmentRepository.cs

@@ -1,16 +1,52 @@
 namespace Domain.Ports.Repositories;
 
 using Domain.Entities;
+using Domain.ReadModels;
 
+/// <summary>
+/// Repository interface for enrollment persistence operations.
+/// </summary>
 public interface IEnrollmentRepository
 {
+    /// <summary>
+    /// Gets an enrollment by its unique identifier.
+    /// </summary>
     Task<Enrollment?> GetByIdAsync(int id, CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets an enrollment by student and subject combination.
+    /// </summary>
     Task<Enrollment?> GetByStudentAndSubjectAsync(int studentId, int subjectId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets all enrollments for a specific student with related entities.
+    /// </summary>
     Task<IReadOnlyList<Enrollment>> GetByStudentIdAsync(int studentId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets all enrollments for a specific subject with related entities.
+    /// </summary>
     Task<IReadOnlyList<Enrollment>> GetBySubjectIdAsync(int subjectId, CancellationToken ct = default);
-    Task<IReadOnlyList<Student>> GetClassmatesAsync(int studentId, int subjectId, CancellationToken ct = default);
+
-    Task<IReadOnlyDictionary<int, IReadOnlyList<Student>>> GetClassmatesBatchAsync(
+    /// <summary>
+    /// Gets classmates (other students) enrolled in the same subject.
+    /// </summary>
+    Task<IReadOnlyList<ClassmateInfo>> GetClassmatesAsync(int studentId, int subjectId, CancellationToken ct = default);
+
+    /// <summary>
+    /// Batch query to get classmates for multiple subjects in a single database call.
+    /// Eliminates N+1 query problem when loading classmates for all enrolled subjects.
+    /// </summary>
+    Task<IReadOnlyDictionary<int, IReadOnlyList<ClassmateInfo>>> GetClassmatesBatchAsync(
         int studentId, IEnumerable<int> subjectIds, CancellationToken ct = default);
+
+    /// <summary>
+    /// Adds a new enrollment to the context.
+    /// </summary>
     void Add(Enrollment enrollment);
+
+    /// <summary>
+    /// Removes an enrollment from the context.
+    /// </summary>
     void Delete(Enrollment enrollment);
 }

src/backend/Domain/Ports/Repositories/IProfessorRepository.cs

@@ -3,13 +3,29 @@ namespace Domain.Ports.Repositories;
 using Domain.Entities;
 using System.Linq.Expressions;
 
+/// <summary>
+/// Repository interface for professor persistence operations.
+/// </summary>
 public interface IProfessorRepository
 {
+    /// <summary>
+    /// Gets a professor by ID without related entities.
+    /// </summary>
     Task<Professor?> GetByIdAsync(int id, CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets all professors without related entities.
+    /// </summary>
     Task<IReadOnlyList<Professor>> GetAllAsync(CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets all professors with their subjects.
+    /// </summary>
     Task<IReadOnlyList<Professor>> GetAllWithSubjectsAsync(CancellationToken ct = default);
 
-    // Projection support
+    /// <summary>
+    /// Gets all professors with custom projection to avoid over-fetching.
+    /// </summary>
     Task<IReadOnlyList<TResult>> GetAllProjectedAsync<TResult>(
         Expression<Func<Professor, TResult>> selector,
         CancellationToken ct = default);

src/backend/Domain/Ports/Repositories/IStudentRepository.cs

@@ -3,32 +3,85 @@ namespace Domain.Ports.Repositories;
 using Domain.Entities;
 using System.Linq.Expressions;
 
+/// <summary>
+/// Repository interface for student persistence operations.
+/// </summary>
 public interface IStudentRepository
 {
+    /// <summary>
+    /// Gets a student by ID without related entities.
+    /// </summary>
     Task<Student?> GetByIdAsync(int id, CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets a student by ID with enrollments and related subjects.
+    /// </summary>
     Task<Student?> GetByIdWithEnrollmentsAsync(int id, CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets all students without related entities.
+    /// </summary>
     Task<IReadOnlyList<Student>> GetAllAsync(CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets all students with enrollments and related subjects.
+    /// </summary>
     Task<IReadOnlyList<Student>> GetAllWithEnrollmentsAsync(CancellationToken ct = default);
+
+    /// <summary>
+    /// Checks if a student exists by ID.
+    /// </summary>
     Task<bool> ExistsAsync(int id, CancellationToken ct = default);
+
+    /// <summary>
+    /// Checks if an email is already in use, optionally excluding a student.
+    /// </summary>
+    /// <param name="email">The email to check.</param>
+    /// <param name="excludeId">Optional student ID to exclude from the check.</param>
+    /// <param name="ct">Cancellation token.</param>
     Task<bool> EmailExistsAsync(string email, int? excludeId = null, CancellationToken ct = default);
 
-    // Projection support for optimized queries
+    /// <summary>
+    /// Gets all students with custom projection to avoid over-fetching.
+    /// </summary>
     Task<IReadOnlyList<TResult>> GetAllProjectedAsync<TResult>(
         Expression<Func<Student, TResult>> selector,
         CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets a student by ID with custom projection.
+    /// </summary>
     Task<TResult?> GetByIdProjectedAsync<TResult>(
         int id,
         Expression<Func<Student, TResult>> selector,
         CancellationToken ct = default);
 
-    // Keyset pagination (more efficient than OFFSET)
+    /// <summary>
+    /// Gets students with keyset pagination (more efficient than OFFSET).
+    /// </summary>
+    /// <param name="selector">Projection selector.</param>
+    /// <param name="afterId">Cursor: ID after which to start.</param>
+    /// <param name="pageSize">Number of items per page.</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>Items, next cursor (if any), and total count.</returns>
     Task<(IReadOnlyList<TResult> Items, int? NextCursor, int TotalCount)> GetPagedProjectedAsync<TResult>(
         Expression<Func<Student, TResult>> selector,
         int? afterId = null,
         int pageSize = 10,
         CancellationToken ct = default);
 
+    /// <summary>
+    /// Adds a new student to the context.
+    /// </summary>
     void Add(Student student);
+
+    /// <summary>
+    /// Marks a student as modified in the context.
+    /// </summary>
     void Update(Student student);
+
+    /// <summary>
+    /// Removes a student from the context.
+    /// </summary>
     void Delete(Student student);
 }

src/backend/Domain/Ports/Repositories/ISubjectRepository.cs

@@ -3,15 +3,39 @@ namespace Domain.Ports.Repositories;
 using Domain.Entities;
 using System.Linq.Expressions;
 
+/// <summary>
+/// Repository interface for subject persistence operations.
+/// </summary>
 public interface ISubjectRepository
 {
+    /// <summary>
+    /// Gets a subject by ID without related entities.
+    /// </summary>
     Task<Subject?> GetByIdAsync(int id, CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets a subject by ID with its professor.
+    /// </summary>
     Task<Subject?> GetByIdWithProfessorAsync(int id, CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets all subjects without related entities.
+    /// </summary>
     Task<IReadOnlyList<Subject>> GetAllAsync(CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets all subjects with their professors.
+    /// </summary>
     Task<IReadOnlyList<Subject>> GetAllWithProfessorsAsync(CancellationToken ct = default);
+
+    /// <summary>
+    /// Gets subjects available for a student (not enrolled, no professor conflict).
+    /// </summary>
     Task<IReadOnlyList<Subject>> GetAvailableForStudentAsync(int studentId, CancellationToken ct = default);
 
-    // Projection support
+    /// <summary>
+    /// Gets all subjects with custom projection to avoid over-fetching.
+    /// </summary>
     Task<IReadOnlyList<TResult>> GetAllProjectedAsync<TResult>(
         Expression<Func<Subject, TResult>> selector,
         CancellationToken ct = default);

src/backend/Domain/ReadModels/ClassmateInfo.cs

@@ -0,0 +1,9 @@
+namespace Domain.ReadModels;
+
+/// <summary>
+/// Minimal read-only projection of a student for classmate queries.
+/// Used to avoid loading full Student entities when only Id and Name are needed.
+/// </summary>
+/// <param name="StudentId">The student's unique identifier.</param>
+/// <param name="Name">The student's full name.</param>
+public readonly record struct ClassmateInfo(int StudentId, string Name);

src/backend/Domain/Services/EnrollmentDomainService.cs

@@ -3,8 +3,20 @@ namespace Domain.Services;
 using Domain.Entities;
 using Domain.Exceptions;
 
+/// <summary>
+/// Domain service for enrollment business rules.
+/// Contains validation logic that spans multiple aggregates.
+/// </summary>
 public class EnrollmentDomainService
 {
+    /// <summary>
+    /// Validates that a student can enroll in a subject.
+    /// </summary>
+    /// <param name="student">The student attempting to enroll.</param>
+    /// <param name="subject">The subject to enroll in.</param>
+    /// <exception cref="MaxEnrollmentsExceededException">Student has reached max enrollments.</exception>
+    /// <exception cref="SameProfessorConstraintException">Student already has a subject with this professor.</exception>
+    /// <exception cref="DuplicateEnrollmentException">Student is already enrolled in this subject.</exception>
     public void ValidateEnrollment(Student student, Subject subject)
     {
         if (!student.CanEnroll())
@@ -17,6 +29,12 @@ public class EnrollmentDomainService
             throw new DuplicateEnrollmentException(student.Id, subject.Id);
     }
 
+    /// <summary>
+    /// Creates an enrollment after validating business rules.
+    /// </summary>
+    /// <param name="student">The student to enroll.</param>
+    /// <param name="subject">The subject to enroll in.</param>
+    /// <returns>The created enrollment.</returns>
     public Enrollment CreateEnrollment(Student student, Subject subject)
     {
         ValidateEnrollment(student, subject);
@@ -26,4 +44,30 @@ public class EnrollmentDomainService
 
         return enrollment;
     }
+
+    /// <summary>
+    /// Checks if a subject is available for enrollment by a student.
+    /// Returns availability status and reason without throwing exceptions.
+    /// </summary>
+    /// <param name="student">The student checking availability.</param>
+    /// <param name="subject">The subject to check.</param>
+    /// <returns>A tuple with availability status and optional reason if unavailable.</returns>
+    public (bool IsAvailable, string? Reason) CheckSubjectAvailability(
+        Student student,
+        Subject subject)
+    {
+        // Already enrolled in this subject
+        if (student.Enrollments.Any(e => e.SubjectId == subject.Id))
+            return (false, "Already enrolled");
+
+        // Maximum enrollments reached
+        if (!student.CanEnroll())
+            return (false, "Maximum 3 subjects reached");
+
+        // Already has a subject with this professor
+        if (student.HasProfessor(subject.ProfessorId))
+            return (false, "Already have a subject with this professor");
+
+        return (true, null);
+    }
 }

tests/Application.Tests/Application.Tests.csproj

@@ -25,6 +25,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\src\backend\Application\Application.csproj" />
+    <ProjectReference Include="..\Common\Common.csproj" />
   </ItemGroup>
 
 </Project>

tests/Application.Tests/Enrollments/GetClassmatesQueryTests.cs

@@ -4,6 +4,7 @@ using Application.Enrollments.Queries;
 using Domain.Entities;
 using Domain.Exceptions;
 using Domain.Ports.Repositories;
+using Domain.ReadModels;
 using Domain.ValueObjects;
 using FluentAssertions;
 using NSubstitute;
@@ -84,12 +85,10 @@ public class GetClassmatesQueryTests
         _studentRepository.GetByIdWithEnrollmentsAsync(1, Arg.Any<CancellationToken>())
             .Returns(student);
 
-        var classmate1 = new Student("Jane Doe", Email.Create("jane@test.com"));
+        var classmate1 = new ClassmateInfo(2, "Jane Doe");
-        var classmate2 = new Student("Bob Smith", Email.Create("bob@test.com"));
+        var classmate2 = new ClassmateInfo(3, "Bob Smith");
-        SetEntityId(classmate1, 2);
-        SetEntityId(classmate2, 3);
 
-        var batchResult = new Dictionary<int, IReadOnlyList<Student>>
+        var batchResult = new Dictionary<int, IReadOnlyList<ClassmateInfo>>
         {
             [1] = [classmate1],
             [2] = [classmate2]
@@ -129,7 +128,7 @@ public class GetClassmatesQueryTests
         _studentRepository.GetByIdWithEnrollmentsAsync(1, Arg.Any<CancellationToken>())
             .Returns(student);
 
-        var emptyBatchResult = new Dictionary<int, IReadOnlyList<Student>>();
+        var emptyBatchResult = new Dictionary<int, IReadOnlyList<ClassmateInfo>>();
         _enrollmentRepository.GetClassmatesBatchAsync(1, Arg.Any<IEnumerable<int>>(), Arg.Any<CancellationToken>())
             .Returns(emptyBatchResult);
 

tests/Application.Tests/Subjects/SubjectQueriesTests.cs

@@ -92,13 +92,15 @@ public class GetAvailableSubjectsQueryTests
 {
     private readonly IStudentRepository _studentRepository;
     private readonly ISubjectRepository _subjectRepository;
+    private readonly Domain.Services.EnrollmentDomainService _enrollmentDomainService;
     private readonly GetAvailableSubjectsHandler _handler;
 
     public GetAvailableSubjectsQueryTests()
     {
         _studentRepository = Substitute.For<IStudentRepository>();
         _subjectRepository = Substitute.For<ISubjectRepository>();
-        _handler = new GetAvailableSubjectsHandler(_studentRepository, _subjectRepository);
+        _enrollmentDomainService = new Domain.Services.EnrollmentDomainService();
+        _handler = new GetAvailableSubjectsHandler(_studentRepository, _subjectRepository, _enrollmentDomainService);
     }
 
     [Fact]

tests/Common/Builders/EnrollmentBuilder.cs

@@ -0,0 +1,74 @@
+namespace Common.Builders;
+
+using Domain.Entities;
+
+/// <summary>
+/// Builder pattern for creating Enrollment test objects.
+/// </summary>
+public class EnrollmentBuilder
+{
+    private int _id = 1;
+    private int _studentId = 1;
+    private int _subjectId = 1;
+    private Student? _student;
+    private Subject? _subject;
+    private DateTime? _enrolledAt;
+
+    public EnrollmentBuilder WithId(int id)
+    {
+        _id = id;
+        return this;
+    }
+
+    public EnrollmentBuilder WithStudentId(int studentId)
+    {
+        _studentId = studentId;
+        return this;
+    }
+
+    public EnrollmentBuilder WithSubjectId(int subjectId)
+    {
+        _subjectId = subjectId;
+        return this;
+    }
+
+    public EnrollmentBuilder WithStudent(Student student)
+    {
+        _student = student;
+        _studentId = student.Id;
+        return this;
+    }
+
+    public EnrollmentBuilder WithSubject(Subject subject)
+    {
+        _subject = subject;
+        _subjectId = subject.Id;
+        return this;
+    }
+
+    public EnrollmentBuilder WithEnrolledAt(DateTime enrolledAt)
+    {
+        _enrolledAt = enrolledAt;
+        return this;
+    }
+
+    public Enrollment Build()
+    {
+        var enrollment = new Enrollment(_studentId, _subjectId);
+        SetProperty(enrollment, "Id", _id);
+
+        if (_student is not null)
+            SetProperty(enrollment, "Student", _student);
+
+        if (_subject is not null)
+            SetProperty(enrollment, "Subject", _subject);
+
+        if (_enrolledAt.HasValue)
+            SetProperty(enrollment, "EnrolledAt", _enrolledAt.Value);
+
+        return enrollment;
+    }
+
+    private static void SetProperty<T>(T obj, string propertyName, object value) where T : class =>
+        typeof(T).GetProperty(propertyName)!.SetValue(obj, value);
+}

tests/Common/Builders/ProfessorBuilder.cs

@@ -0,0 +1,50 @@
+namespace Common.Builders;
+
+using Domain.Entities;
+
+/// <summary>
+/// Builder pattern for creating Professor test objects.
+/// </summary>
+public class ProfessorBuilder
+{
+    private int _id = 1;
+    private string _name = "Prof. Smith";
+    private readonly List<Subject> _subjects = [];
+
+    public ProfessorBuilder WithId(int id)
+    {
+        _id = id;
+        return this;
+    }
+
+    public ProfessorBuilder WithName(string name)
+    {
+        _name = name;
+        return this;
+    }
+
+    public ProfessorBuilder WithSubject(Subject subject)
+    {
+        _subjects.Add(subject);
+        return this;
+    }
+
+    public Professor Build()
+    {
+        var professor = new Professor(_name);
+        SetProperty(professor, "Id", _id);
+
+        // Access private _subjects field to add test subjects
+        var subjectsList = (List<Subject>)typeof(Professor)
+            .GetField("_subjects", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)!
+            .GetValue(professor)!;
+
+        foreach (var subject in _subjects)
+            subjectsList.Add(subject);
+
+        return professor;
+    }
+
+    private static void SetProperty<T>(T obj, string propertyName, object value) where T : class =>
+        typeof(T).GetProperty(propertyName)!.SetValue(obj, value);
+}

tests/Common/Builders/StudentBuilder.cs

@@ -0,0 +1,78 @@
+namespace Common.Builders;
+
+using Domain.Entities;
+using Domain.ValueObjects;
+
+/// <summary>
+/// Builder pattern for creating Student test objects.
+/// Encapsulates reflection-based ID setting for cleaner tests.
+/// </summary>
+public class StudentBuilder
+{
+    private int _id = 1;
+    private string _name = "John Doe";
+    private string _email = "john@test.com";
+    private readonly List<(Subject Subject, int EnrollmentId)> _enrollments = [];
+
+    public StudentBuilder WithId(int id)
+    {
+        _id = id;
+        return this;
+    }
+
+    public StudentBuilder WithName(string name)
+    {
+        _name = name;
+        return this;
+    }
+
+    public StudentBuilder WithEmail(string email)
+    {
+        _email = email;
+        return this;
+    }
+
+    public StudentBuilder WithEnrollment(Subject subject, int enrollmentId = 0)
+    {
+        _enrollments.Add((subject, enrollmentId));
+        return this;
+    }
+
+    public StudentBuilder WithEnrollments(int count)
+    {
+        for (int i = 0; i < count; i++)
+        {
+            var subject = new SubjectBuilder()
+                .WithId(i + 100)
+                .WithName($"Subject{i}")
+                .WithProfessorId(i + 1)
+                .WithProfessor(new ProfessorBuilder()
+                    .WithId(i + 1)
+                    .WithName($"Prof{i}")
+                    .Build())
+                .Build();
+            _enrollments.Add((subject, i + 1));
+        }
+        return this;
+    }
+
+    public Student Build()
+    {
+        var student = new Student(_name, Email.Create(_email));
+        SetProperty(student, "Id", _id);
+
+        foreach (var (subject, enrollmentId) in _enrollments)
+        {
+            var enrollment = new Enrollment(student.Id, subject.Id);
+            if (enrollmentId > 0)
+                SetProperty(enrollment, "Id", enrollmentId);
+            SetProperty(enrollment, "Subject", subject);
+            student.AddEnrollment(enrollment);
+        }
+
+        return student;
+    }
+
+    private static void SetProperty<T>(T obj, string propertyName, object value) where T : class =>
+        typeof(T).GetProperty(propertyName)!.SetValue(obj, value);
+}

tests/Common/Builders/SubjectBuilder.cs

@@ -0,0 +1,54 @@
+namespace Common.Builders;
+
+using Domain.Entities;
+
+/// <summary>
+/// Builder pattern for creating Subject test objects.
+/// Encapsulates reflection-based ID and navigation property setting.
+/// </summary>
+public class SubjectBuilder
+{
+    private int _id = 1;
+    private string _name = "Mathematics";
+    private int _professorId = 1;
+    private Professor? _professor;
+
+    public SubjectBuilder WithId(int id)
+    {
+        _id = id;
+        return this;
+    }
+
+    public SubjectBuilder WithName(string name)
+    {
+        _name = name;
+        return this;
+    }
+
+    public SubjectBuilder WithProfessorId(int professorId)
+    {
+        _professorId = professorId;
+        return this;
+    }
+
+    public SubjectBuilder WithProfessor(Professor professor)
+    {
+        _professor = professor;
+        _professorId = professor.Id;
+        return this;
+    }
+
+    public Subject Build()
+    {
+        var subject = new Subject(_name, _professorId);
+        SetProperty(subject, "Id", _id);
+
+        if (_professor is not null)
+            SetProperty(subject, "Professor", _professor);
+
+        return subject;
+    }
+
+    private static void SetProperty<T>(T obj, string propertyName, object value) where T : class =>
+        typeof(T).GetProperty(propertyName)!.SetValue(obj, value);
+}

tests/Common/Common.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\backend\Domain\Domain.csproj" />
+  </ItemGroup>
+
+</Project>

tests/Domain.Tests/Domain.Tests.csproj

@@ -24,6 +24,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\src\backend\Domain\Domain.csproj" />
+    <ProjectReference Include="..\Common\Common.csproj" />
   </ItemGroup>
 
 </Project>