This pull request introduces major improvements to the backend of the library system, focusing on implementing and testing the book loan and return functionality. It adds a new router for handling loans, updates the database schema and initialization, improves logging and environment variable handling, and introduces comprehensive automated tests for the loans API. Additionally, it refactors some schema files and updates dependencies.

The most important changes are:

Loan API Implementation and Testing

• Added a new router_loans FastAPI router with endpoints for creating loans and returning books, including logic for updating book status, calculating and accumulating fines, and blocking users with excessive fines.
• Added comprehensive tests for the loans endpoints in test_loans.py, covering scenarios like on-time returns, overdue fines, user blocking, missing loans or book copies, and correct status updates.

These changes collectively provide a robust foundation for managing book loans, handling edge cases, and ensuring the API is well-tested and maintainable.