feat: auto-chain vision backfill + face recluster after scan, show progress in UI

Scan now automatically queues backfill_vision (+90s) and recluster_faces
(+300s) after dispatching folder scans. Face extraction also schedules a
debounced recluster via Redis so incremental file-watcher imports get
clustered without manual intervention.

The ScanProgress widget now tracks worker queue activity beyond the scan
phase, showing a "Processing Photos" indicator with vision queue counts
while background tasks (embeddings, faces, tags, OCR) are running.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/tasks/vision.py

@@ -348,13 +348,40 @@ def _save_faces(photo_id: str, faces) -> dict:
 
     if faces:
         logger.info("Extracted %d verified face(s) from photo %s", len(faces), photo_id)
+        _schedule_recluster_debounced()
     return {'status': 'success', 'photo_id': photo_id, 'faces': len(faces)}
 
 
+RECLUSTER_DEBOUNCE_KEY = "mule:recluster_faces:pending"
+RECLUSTER_DELAY = 120  # seconds after last face extraction
+
+
+def _schedule_recluster_debounced():
+    """Schedule a recluster_faces run, debounced so rapid-fire face
+    extractions don't spawn hundreds of redundant cluster jobs."""
+    try:
+        import redis as _redis
+        r = _redis.from_url(settings.redis_url)
+        already_pending = r.set(RECLUSTER_DEBOUNCE_KEY, "1",
+                                ex=RECLUSTER_DELAY, nx=True)
+        if already_pending:
+            recluster_faces.apply_async(countdown=RECLUSTER_DELAY)
+            logger.info("Scheduled debounced recluster_faces in %ds", RECLUSTER_DELAY)
+    except Exception as e:
+        logger.debug("recluster debounce check failed: %s", e)
+
+
 @shared_task(name='recluster_faces', queue='vision')
 def recluster_faces():
     """Run DBSCAN clustering over all face embeddings and assign/create
     Tag(kind=face_cluster) entries."""
+    # Clear debounce key so new face extractions can schedule another round.
+    try:
+        import redis as _redis
+        _redis.from_url(settings.redis_url).delete(RECLUSTER_DEBOUNCE_KEY)
+    except Exception:
+        pass
+
     if not settings.vision.enabled or not settings.vision.faces.enabled:
         return {'status': 'skipped', 'reason': 'faces disabled'}