docs: add project documentation and env template
Environment:
- .env.example template with all configuration variables
- Database, API, GraphQL, and frontend settings
Documentation:
- Architecture Decision Records (ADR-001 to ADR-004)
- Deployment guide with Docker and K8s instructions
- OWASP security checklist
- Code review checklist
- Activity plan and deliverables
Architecture diagrams (PlantUML):
- Use cases, domain model, sequence diagrams
- Component, ER, state, and deployment diagrams
- C4 context diagram
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