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/tests/crud/test_user.py

@@ -1,6 +1,6 @@
 import pytest
 
-from app.crud.user import user as user_crud
+from app.crud.user import user_crud as user_crud
 from app.models.user import User
 from app.schemas.users import UserCreate, UserUpdate