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/scan.py

@@ -409,6 +409,7 @@ async def _scan_all_source_roots_async():
     still hit Settings → Re-detect duplicates to force a fresh pass.
     """
     from app.tasks.thumbs import regroup_duplicates_task
+    from app.tasks.vision import backfill_vision, recluster_faces
 
     async with AsyncSessionLocal() as session:
         result = await session.execute(
@@ -433,6 +434,21 @@ async def _scan_all_source_roots_async():
         except Exception as e:
             logger.warning(f"Could not queue post-scan regroup: {e}")
 
+        # 90s lets thumbnails finish so photos reach processing_status
+        # 'completed', which backfill_vision uses as its filter.
+        try:
+            backfill_vision.apply_async(countdown=90)
+        except Exception as e:
+            logger.warning(f"Could not queue post-scan vision backfill: {e}")
+
+        # 300s gives face extraction time to run before reclustering.
+        # Fires even if some faces are still in-flight — the task is
+        # idempotent and the user can re-trigger from Settings.
+        try:
+            recluster_faces.apply_async(countdown=300)
+        except Exception as e:
+            logger.warning(f"Could not queue post-scan face recluster: {e}")
+
 
 @shared_task(name='watch_folders')
 def watch_folders():