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/requirements.txt

@@ -41,4 +41,10 @@ requests>=2.32.0
 black>=24.3.0
 isort>=5.13.2
 flake8>=7.0.0
-mypy>=1.8.0
+mypy>=1.8.0
+
+# Security
+python-jose==3.4.0
+bcrypt==4.2.1
+cryptography==44.0.1
+passlib==1.7.4