feat(phase 4): vision fetches NC previews; stop writing /data/thumbs

Last consumer of the on-disk thumbnail pipeline was the vision
worker reading /data/thumbs/{id}/medium.webp. Now it asks Nextcloud
for a 640px preview (the same edge size the old thumb used) and
decodes the bytes in-memory — no disk dependency.

- nextcloud_dav.get_preview_bytes: sync sibling of get_preview_async,
  for the celery vision worker (which is sync).
- vision._load_thumb: tries NC preview first; transitional disk
  fallback stays for rows still indexed during the rollout.
- thumbs.WORKER_THUMB_SIZES = set() — generate_thumbnails still runs
  the decode + pHash side-effect (perceptual dedup is mule-only and
  needs original-resolution pixels) but no longer writes thumbnail
  files.

The HTTP thumbnail endpoint's disk fallback path stays in place
unchanged: for NC-404 cases (e.g. iPhone JPEGs mis-extensioned as
.DNG), inline Pillow regeneration still writes a tiny per-photo
file so subsequent requests are fast. That path is rare and the
files are small.

Disk impact: /data/thumbs currently has ~22k medium.webp totaling
~1 GB. They'll stop being read after the worker-vision container
restarts, but no automatic delete — purge with the same find
pattern used for small/large reclaim when ready:

    find /data/thumbs -name "medium.webp" -delete

backend/app/tasks/vision.py

@@ -45,7 +45,76 @@ def _get_sync_session() -> Session:
     return sessionmaker(bind=_get_sync_engine())()
 
 
+# CLIP was trained against 640px medium thumbs that the on-disk
+# pipeline used to produce. Now we ask Nextcloud's preview endpoint
+# for the same edge size so the classifier sees the same input
+# distribution.
+_VISION_PREVIEW_PX = 640
+
+
 def _load_thumb(photo_id: str, size: str = "medium") -> np.ndarray | None:
+    """Load the photo's RGB pixels into a numpy array for inference.
+
+    Primary path: ask Nextcloud's `/index.php/core/preview` for a
+    640px preview via the sync helper. Replaces the disk read of
+    `/data/thumbs/{photo_id}/medium.webp` so the on-disk pipeline
+    can retire entirely.
+
+    Disk fallback (transitional): if NC has no preview or no
+    credentials, look for the medium.webp the old pipeline wrote.
+    Goes dead once `generate_thumbnails` stops writing files.
+    """
+    from io import BytesIO
+    from app.models import Photo
+    from app.models.user import User
+    from app.services.nextcloud_dav import get_preview_bytes
+
+    user_id: str | None = None
+    fileid: int | None = None
+    session = _get_sync_session()
+    try:
+        row = session.execute(
+            select(Photo.user_id, Photo.nextcloud_fileid).where(
+                Photo.id == photo_id
+            )
+        ).one_or_none()
+        if row:
+            user_id, fileid = row[0], row[1]
+    finally:
+        session.close()
+
+    if user_id and fileid:
+        session = _get_sync_session()
+        try:
+            owner = session.execute(
+                select(User).where(User.id == user_id)
+            ).scalar_one_or_none()
+        finally:
+            session.close()
+        if owner is not None and owner.nextcloud_app_password_enc:
+            try:
+                body = get_preview_bytes(
+                    owner, fileid, _VISION_PREVIEW_PX, _VISION_PREVIEW_PX,
+                )
+            except Exception as e:
+                logger.warning(
+                    "NC preview fetch failed for %s: %s", photo_id, e
+                )
+                body = None
+            if body:
+                try:
+                    img = Image.open(BytesIO(body)).convert("RGB")
+                    img.load()
+                    arr = np.array(img)
+                    img.close()
+                    return arr
+                except Exception as e:
+                    logger.warning(
+                        "NC preview decode failed for %s: %s", photo_id, e
+                    )
+
+    # Legacy disk fallback — transitional, dead once thumbs.py stops
+    # writing /data/thumbs.
     thumb_base = Path("/data/thumbs")
     thumb_path = thumb_base / photo_id / f"{size}.webp"
     if not thumb_path.exists():
@@ -53,7 +122,7 @@ def _load_thumb(photo_id: str, size: str = "medium") -> np.ndarray | None:
         if matches:
             thumb_path = matches[0]
         else:
-            logger.warning("Thumbnail not found: %s", thumb_path)
+            logger.warning("Thumbnail not found anywhere for %s", photo_id)
             return None
     try:
         img = Image.open(thumb_path).convert("RGB")