mule-image was generating and storing three WebP sizes per photo in
/data/thumbs while Nextcloud already keeps its own previews for the
same source files. Frontend thumbnail requests now proxy NC's
/index.php/core/preview keyed by the photo's Nextcloud fileid,
authenticated with the owner's encrypted app password.
- new column photos.nextcloud_fileid (alembic 0018) plus an index
- get_preview_async + fetch_fileid helpers in nextcloud_dav.py
- thumb route proxies NC primary, falls back to /data/thumbs (legacy
rows / NC unreachable) so a single-file revert restores the old path
- extract_metadata caches the fileid on first run for new photos
- generate_thumbnails now writes only medium since the vision worker
still loads it from disk; small + large drop out of the worker path
- backend/scripts/backfill_nextcloud_fileid.py for one-shot population
of existing rows: docker exec mulita-backend python -m scripts.backfill_nextcloud_fileid
X-Mule-Thumb-Source response header marks each request 'nextcloud' or
'disk' for observability while the rollout settles.
