feat(thumbs): proxy Nextcloud previews instead of duplicating the cache

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.

backend/app/services/metadata.py

@@ -204,11 +204,39 @@ async def _extract_metadata_async(photo_id: str):
                 select(Photo).where(Photo.id == photo_id)
             )
             photo = result.scalar_one_or_none()
-            
+
             if not photo:
                 logger.error(f"Photo not found: {photo_id}")
                 return {'status': 'error', 'message': 'Photo not found'}
-            
+
+            # Cache Nextcloud's numeric fileid on the row so the thumbnail
+            # handler can proxy /index.php/core/preview without doing a
+            # PROPFIND per request. PROPFIND blocks for ~50ms; tolerable
+            # because extract_metadata already does seconds of ExifTool
+            # work. Failures are silent — the thumb handler falls back
+            # to its on-disk path when the column is NULL.
+            if photo.nextcloud_fileid is None and photo.user_id:
+                from app.models.user import User
+                from app.services.nextcloud_dav import (
+                    fetch_fileid, is_nextcloud_path,
+                )
+                if photo.filepath and is_nextcloud_path(photo.filepath):
+                    owner = (
+                        await session.execute(
+                            select(User).where(User.id == photo.user_id)
+                        )
+                    ).scalar_one_or_none()
+                    if owner is not None and owner.nextcloud_app_password_enc:
+                        try:
+                            fid = fetch_fileid(owner, photo.filepath)
+                        except Exception as e:
+                            logger.warning(
+                                "fileid lookup failed for %s: %s", photo_id, e
+                            )
+                            fid = None
+                        if fid is not None:
+                            photo.nextcloud_fileid = fid
+
             # Check if file exists
             if not Path(photo.filepath).exists():
                 logger.error(f"File not found: {photo.filepath}")