Refactor database module and add testing utilities

Simplify database module by re-organizing engine creation, session handling, and removing redundant methods. Introduce SQLite compatibility for testing and add a utility module for test database setup and teardown. Integrate initial unit tests for user models and update dependencies for security and testing.

backend/pytest.ini

@@ -0,0 +1,9 @@
+[pytest]
+env =
+    IS_TEST=True
+testpaths = tests
+python_files = test_*.py
+addopts = --disable-warnings
+markers =
+    sqlite: marks tests that should run on SQLite (mocked).
+    postgres: marks tests that require a real PostgreSQL database.