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/duplicates.py

@@ -125,7 +125,7 @@ async def regroup_duplicates(
 
     Idempotent — safe to call as often as you like. Returns a summary dict.
     """
-    embedder_model = settings.mulita_config.vision.embedder.name
+    embedder_model = settings.vision.embedder.name
 
     async with AsyncSessionLocal() as session:
         # Pull all visible photos with a phash or embedding.
@@ -210,7 +210,7 @@ async def incremental_regroup(
     Much faster than a full regroup for post-scan updates:
     O(new × log N) via HNSW instead of O(N²).
     """
-    embedder_model = settings.mulita_config.vision.embedder.name
+    embedder_model = settings.vision.embedder.name
 
     async with AsyncSessionLocal() as session:
         # If no watermark, fall back to full regroup.