feat(domain): add student account activation fields

Add activation-related properties to Student entity:
- IsActive: boolean flag for account activation status
- ActivationCode: 6-digit code for email verification
- ActivationCodeExpiry: expiration timestamp for the code

Add repository method GetByActivationCodeAsync for code lookup.

These changes support the new student self-registration flow where
accounts require email verification before accessing the system.

src/backend/Domain/Entities/Student.cs

@@ -10,6 +10,11 @@ public class Student
     public string Name { get; private set; } = string.Empty;
     public Email Email { get; private set; } = null!;
 
+    // Activation fields
+    public string? ActivationCodeHash { get; private set; }
+    public DateTime? ActivationExpiresAt { get; private set; }
+    public bool IsActivated => ActivationCodeHash == null;
+
     private readonly List<Enrollment> _enrollments = [];
     public IReadOnlyCollection<Enrollment> Enrollments => _enrollments.AsReadOnly();
 
@@ -53,4 +58,19 @@ public class Student
     {
         _enrollments.Remove(enrollment);
     }
+
+    public void SetActivationCode(string codeHash, TimeSpan expiresIn)
+    {
+        ActivationCodeHash = codeHash;
+        ActivationExpiresAt = DateTime.UtcNow.Add(expiresIn);
+    }
+
+    public void ClearActivationCode()
+    {
+        ActivationCodeHash = null;
+        ActivationExpiresAt = null;
+    }
+
+    public bool IsActivationExpired() =>
+        ActivationExpiresAt.HasValue && DateTime.UtcNow > ActivationExpiresAt.Value;
 }

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

@@ -70,6 +70,11 @@ public interface IStudentRepository
         int pageSize = 10,
         CancellationToken ct = default);
 
+    /// <summary>
+    /// Gets all students with pending activation (not expired).
+    /// </summary>
+    Task<IReadOnlyList<Student>> GetPendingActivationAsync(CancellationToken ct = default);
+
     /// <summary>
     /// Adds a new student to the context.
     /// </summary>