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/services/scanner.py

@@ -78,17 +78,26 @@ async def bootstrap_default_source_root() -> None:
 
 
 async def start_initial_scan():
-    """Start the initial library scan.
+    """Start the initial library scan and optionally the file watcher.
 
-    NOTE: the folder watcher (watch_folders task) is intentionally NOT
-    dispatched here. It's an infinite loop celery task and every backend
-    restart was queuing a new instance, eventually pinning every worker
-    and starving scan_folder dispatches. Re-enabling it needs a Redis
-    lock or a dedicated long-running container — until then the user
-    triggers scans manually via "Scan all folders".
+    The file watcher uses a Redis lock to ensure only one instance runs
+    across all workers, so it's safe to dispatch on every startup — only
+    the first one will actually watch, the rest exit immediately.
     """
     try:
         scan_all_source_roots.delay()
         logger.info("Initial scan queued successfully")
     except Exception as e:
         logger.error(f"Failed to start initial scan: {e}")
+
+    # Start the file watcher if enabled in config.
+    from app.config import settings
+    if settings.scanner.watch:
+        try:
+            from app.tasks.scan import watch_folders
+            # Countdown gives the initial scan time to register source roots
+            # before the watcher tries to load them.
+            watch_folders.apply_async(countdown=10)
+            logger.info("File watcher queued (Redis-locked, single instance)")
+        except Exception as e:
+            logger.warning(f"Could not queue file watcher: {e}")