♻️(backend) replace Django permissions with feature flag decorator

Refactor feature flag mechanism from Django permission classes to custom
decorator that returns 404 Not Found when features are disabled instead
of exposing API structure through permission errors.

Improves security by preventing information disclosure about disabled
features and provides more appropriate response semantics. Custom
decorator approach is better suited for feature toggling than Django's
permission system which is designed for authorization.

src/backend/core/tests/recording/test_api_recordings_storage_hook.py

@@ -77,7 +77,8 @@ def test_save_recording_permission_needed(settings, client):
         HTTP_AUTHORIZATION="Bearer testAuthToken",
     )
 
-    assert response.status_code == 403
+    assert response.status_code == 404
+    assert response.json() == {"detail": "Not found."}
 
 
 def test_save_recording_parsing_error(recording_settings, mock_get_parser, client):