Refactor CRUD module imports and usage for clarity.

Replaces ambiguous shorthand references like `event` and `user` with more descriptive names such as `event_crud` and `user_crud`. Updates imports, function calls, tests, and other references across the codebase to maintain consistency. This improves code readability and reduces potential confusion.

backend/app/crud/user.py

@@ -53,4 +53,4 @@ class CRUDUser(CRUDBase[User, UserCreate, UserUpdate]):
 
 
 # Create a singleton instance for use across the application
-user = CRUDUser(User)
+user_crud = CRUDUser(User)