Add and extend test coverage for models and their methods

Enhanced test coverage includes `repr` methods, model functionality, and validation logic for key models like `GiftItem`, `GiftCategory`, `EventMedia`, `RSVP`, and `GiftPurchase`. Refactored and added fixtures to support comprehensive testing scenarios. Addresses validation for gift reordering and updates `EventMedia` representation format for consistency.

backend/tests/models/test_event_media.py

@@ -101,4 +101,12 @@ def test_media_metadata(event_media_fixture):
 
     # Update and check
     event_media_fixture.media_metadata = {"resolution": "1280x720"}
-    assert event_media_fixture.media_metadata == {"resolution": "1280x720"}
+    assert event_media_fixture.media_metadata == {"resolution": "1280x720"}
+
+def test_event_media_repr(event_media_fixture):
+    # Act
+    result = repr(event_media_fixture)
+
+    # Assert
+    expected = f"<EventMedia {event_media_fixture.original_filename} ({event_media_fixture.media_type.value}) for event={event_media_fixture.event_id}>"
+    assert result == expected