Enhance user management, improve API structure, add database optimizations, and update Docker setup

- Introduced endpoints for user management, including CRUD operations, pagination, and password management.
- Added new schema validations for user updates, password strength, pagination, and standardized error responses.
- Integrated custom exception handling for a consistent API error experience.
- Refined CORS settings: restricted methods and allowed headers, added header exposure, and preflight caching.
- Optimized database: added indexes on `is_active` and `is_superuser` fields, updated column types, enforced constraints, and set defaults.
- Updated `Dockerfile` to improve security by using a non-root user and adding a health check for the application.
- Enhanced tests for database initialization, user operations, and exception handling to ensure better coverage.

backend/app/init_db.py

@@ -19,7 +19,7 @@ def init_db(db: Session) -> Optional[UserCreate]:
     """
     # Use default values if not set in environment variables
     superuser_email = settings.FIRST_SUPERUSER_EMAIL or "admin@example.com"
-    superuser_password = settings.FIRST_SUPERUSER_PASSWORD or "admin123"
+    superuser_password = settings.FIRST_SUPERUSER_PASSWORD or "Admin123!Change"
 
     if not settings.FIRST_SUPERUSER_EMAIL or not settings.FIRST_SUPERUSER_PASSWORD:
         logger.warning(