Fix stale browser-cached thumbnails after file content changes during scan. (#6622)

* Fix stale thumbnails after file content changes

When a file's content changed (e.g. after renaming files in a gallery),
the scan handler updated fingerprints but did not bump the entity's
updated_at timestamp. Since thumbnail URLs use updated_at as a cache
buster and are served with immutable/1-year cache headers, browsers
would indefinitely serve the old cached thumbnail.

Update image, scene, and gallery scan handlers to call UpdatePartial
(which sets updated_at to now) whenever file content changes, not only
when a new file association is created.