Add base database manager

[javidahmed64592]

This pull request introduces a foundational database layer to the project using SQLModel, adds configuration support for database settings, and provides comprehensive unit tests for the new functionality. The most important changes are grouped below:

Database Layer Implementation:

• Added sqlmodel as a project dependency in pyproject.toml to enable SQLModel-based database operations.
• Introduced BaseDatabaseManager as an abstract base class for database management, initializing the SQLModel engine and handling database URL configuration.
• Added a module docstring for python_template_server/db/__init__.py.

Configuration Enhancements:

• Added DatabaseConfig model to encapsulate database directory and URL construction logic, and integrated it into the main TemplateServerConfig as a new db field. [1] [2]

Testing Infrastructure:

• Added fixtures in tests/conftest.py for mocking database configuration and temporary database paths, ensuring isolated and reliable tests. [1] [2] [3] [4] [5]
• Created tests/db/test_base_database_manager.py with unit tests for the database manager, including initialization and URL property checks.
• Extended tests/test_models.py with unit tests for the new DatabaseConfig and its integration into TemplateServerConfig. [1] [2] [3] [4]

Test Suite Maintenance:

• Minor test fixture adjustments in tests/test_template_server.py for improved type consistency. [1] [2]