feat: implement repository pattern for improved testability (#681)

[Praizfotos]

Repository Pattern Refactor (BrainTease#681)

📋 Summary

Implements the repository pattern to abstract direct TypeORM access in services for improved testability and consistency across the Brain-Storm backend.

🔧 Changes Made

Repository Infrastructure

• Base Repository Interface: Created BaseRepository<T> with common CRUD operations
• Entity-Specific Interfaces: Added typed repositories for Users, Courses, and Credentials
• TypeORM Implementations: Concrete repository classes that handle TypeORM operations
• Dependency Injection Module: RepositoriesModule with proper token-based DI setup

Service Refactoring

• UsersService: Refactored to use UsersRepository abstraction
• CoursesService: Updated to use CoursesRepository with caching support
• CredentialsService: Migrated to use CredentialsRepository interface

Testing

• Comprehensive Unit Tests: Added mocked repository tests for all refactored services
• Dependency Injection Testing: Verified proper DI setup with Jest mocks
• Behavior Verification: Ensured all existing functionality works with abstractions

🎯 Benefits

Testability

• Services can now be unit tested with mocked repositories
• No more direct database dependencies in service tests
• Faster test execution without TypeORM overhead

Consistency

• Standardized data access patterns across all services
• Single point of control for entity-specific queries
• Easier to maintain and extend data access logic

Flexibility

• Easy to swap implementations (e.g., different ORMs, in-memory stores)
• Repository interfaces define clear contracts
• Better separation of concerns between business logic and data access

📁 File Structure

apps/backend/src/repositories/
├── base-repository.interface.ts
├── users-repository.interface.ts
├── courses-repository.interface.ts  
├── credentials-repository.interface.ts
├── typeorm-users.repository.ts
├── typeorm-courses.repository.ts
├── typeorm-credentials.repository.ts
├── repositories.module.ts
└── index.ts

✅ Acceptance Criteria

• No direct TypeORM access in services (main entities covered)
• Repositories are typed and tested
• Behavior unchanged (abstraction layer transparent)
• Comprehensive unit tests with mocked repositories

🔍 Testing

Run the new repository-based unit tests:

npm test -- users.service.repository.spec.ts
npm test -- courses.service.repository.spec.ts  
npm test -- credentials.service.repository.spec.ts

📝 Notes

• This refactor focuses on the core entities (Users, Courses, Credentials)
• Other services (Auth, Progress, Enrollments) can be migrated in follow-up PRs
• All existing API behavior is preserved through the abstraction layer
• Repository implementations handle both create and update operations seamlessly

Closes BrainTease#681

[Praizfotos]

Closing PR as requested after successful implementation of repository pattern refactor. All acceptance criteria met.

[github-actions]

⚠️ Large PR Warning

This PR changes 689 lines of code (threshold: 500 lines).

Consider breaking this into smaller, more focused PRs for easier review and faster merge times.

[github-actions]

⚠️ Large PR Warning

This PR changes 689 lines of code (threshold: 500 lines).

Consider breaking this into smaller, more focused PRs for easier review and faster merge times.

[Praizfotos]

Closing large PR - will break into smaller focused PRs in the correct BrainTease/Brain-Storm repository