Add tests for the images API

There are backend tests for albums, face clusters, folders, and user preferences, but none for the images routes in backend/app/routes/images.py.
work: Add backend/tests/test_images.py that:
Uses the same style as test_albums.py.
Covers at least:
GET / (and optionally the tagged query).
POST /toggle-favourite (success and “image not found” or error path).
This improves coverage and protects the images API from regressions.