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/event.py

@@ -170,4 +170,4 @@ class CRUDEvent(CRUDBase[Event, EventCreate, EventUpdate]):
 
 
 # Create a singleton instance for use across the application
-event = CRUDEvent(Event)
+event_crud = CRUDEvent(Event)