feat: auto-start file watcher with Redis lock for live import

Re-enable the watchfiles-based folder watcher with a Redis lock to
prevent multiple instances from stacking up across restarts. The
watcher is now automatically dispatched on startup when scanner.watch
is true (default), and only one instance runs at a time.

- Redis lock (SETNX + TTL renewal) ensures single-instance execution
- Graceful exit if another watcher holds the lock
- New POST /maintenance/start-watcher endpoint for manual control
- Fix: use settings.scanner/vision properties instead of mulita_config

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

backend/app/routers/library.py

@@ -840,4 +840,17 @@ async def trigger_backfill_phashes(current_user: User = Depends(get_current_user
         return {"status": "queued"}
     except Exception as e:
         logger.error(f"Backfill queue failed: {e}")
+        return {"status": "error", "message": str(e)}
+
+
+@router.post("/maintenance/start-watcher")
+async def start_file_watcher(current_user: User = Depends(get_current_user)):
+    """Start the filesystem watcher. Uses a Redis lock so only one
+    instance runs at a time — safe to call repeatedly."""
+    from app.tasks.scan import watch_folders
+    try:
+        watch_folders.apply_async(countdown=2)
+        return {"status": "queued"}
+    except Exception as e:
+        logger.error(f"Watcher queue failed: {e}")
         return {"status": "error", "message": str(e)}