fix: disable folder watcher on startup to unblock scan_folder

Diagnosis: every backend restart was dispatching watch_folders.delay()
unconditionally. watch_folders is an infinite-loop celery task
(for changes in watch(*paths)). With CELERYD_CONCURRENCY=4 and several
restarts during dev, all four worker slots ended up pinned by stale
watch_folders instances, leaving zero workers free for scan_folder.
The result: clicking "Scan all folders" successfully queued a task
that then sat in the queue forever, the new /photos/sub folder was
never walked, and the user's newly added photo never appeared.

The watcher was only opportunistically useful and the user already
triggers scans manually. Disabling it removes the foot-gun. Re-
enabling needs:
  - a Redis lock so only one watcher runs at a time
  - or a dedicated long-running container with concurrency=1
  - or a celery beat schedule with a singleton flag

Until then, manual scans work. Cleared the backlog by wiping the
redis broker volume so the stale watch_folders tasks are gone.

Verified: post-fix, scan_folder runs in 0.12s and reports
"Processed 7/7 files. Errors: 0", picking up the previously missing
/photos/sub/Samuel_Colman... file.

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

backend/app/services/scanner.py

@@ -8,7 +8,7 @@ from sqlalchemy import select
 
 from app.database import AsyncSessionLocal
 from app.models import SourceRoot
-from app.tasks.scan import scan_all_source_roots, watch_folders
+from app.tasks.scan import scan_all_source_roots
 from app.config import settings
 
 logger = logging.getLogger(__name__)
@@ -49,15 +49,17 @@ async def bootstrap_default_source_root() -> None:
 
 
 async def start_initial_scan():
-    """Start the initial library scan"""
+    """Start the initial library scan.
+
+    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".
+    """
     try:
-        # Queue scan of all source roots
         scan_all_source_roots.delay()
-
-        # Start folder watcher if configured
-        if settings.scanner.watch:
-            watch_folders.delay()
-
         logger.info("Initial scan queued successfully")
     except Exception as e:
         logger.error(f"Failed to start initial scan: {e}")