feat(tests): overhaul test structure for hybrid database architecture

- Enhanced test configuration in `conftest.py` to support both synchronous and asynchronous testing with SQLModel and py-pglite.
- Introduced comprehensive test fixtures for unit and integration testing, ensuring clean database isolation and performance metrics collection.
- Removed outdated end-to-end and integration tests to streamline the test suite.
- Added new unit tests for PostgreSQL advanced features, including JSON operations and performance monitoring.
- Updated database service tests to reflect the hybrid architecture, ensuring compatibility with both sync and async operations.

Author: kzndotsh