Implement integration tests for API endpoints and database operations

[InfoTCube]

Description:
Create integration tests to verify that the API endpoints, database (PostgreSQL/SQLite), and external services work correctly together.

Tasks:

• Configure TestContainers for PostgreSQL in CI
• Create test database fixtures/seeds
• Test book scanning flow (ISBN → API → save)
• Test friend request lifecycle (send → accept → reject)
• Test borrow request workflow (request → approve → return)
• Test search across friends' libraries
• Test notification generation triggers
• Verify JWT-secured endpoints reject unauthenticated requests

Acceptance Criteria:

• All major API workflows have integration tests
• Tests use isolated test database
• External API calls are mocked or use test keys