Add comprehensive unit tests for API endpoints

[Alen983]

• No test files exist. Adding tests ensures reliability and prevents regressions.
• Relevant part of the codebase: Create backend/tests/ directory with tests for:
• backend/app/routers/auth.py - login, register
• backend/app/routers/leave.py - apply, approve, balance calculations
• backend/app/routers/users.py - role-based access control

Expected outcome:

• Use pytest and httpx for FastAPI testing
• Test happy paths, error cases, and edge cases
• Mock database sessions where appropriate
• Achieve at least 70% code coverage for critical paths
• Add pytest to requirements.txt and create pytest.ini