Update db init

[javidahmed64592]

This pull request improves the structure and documentation of the database manager module in the server template. The changes clarify how to import and subclass the BaseDatabaseManager, and update its usage in tests.

Module structure and imports:

• Added an import for BaseDatabaseManager in python_template_server/db/__init__.py and updated __all__ to make it available for direct import from the db package.
• Updated the import in the test file to use from python_template_server.db import BaseDatabaseManager instead of importing directly from the module.

Documentation and code clarity:

• Improved the class docstring for BaseDatabaseManager to clarify that subclasses must implement the db_url property.
• Updated the docstring for the db_url property to specify that it should use the DatabaseConfig.db_url() method.