feat: settings panel + thumbnail pipeline fixes

- photos.py: stop crashing in FileResponse when a thumb hasn't been
  generated; return a clean 404 with Retry-After so the frontend can
  back off.
- thumbs.py: fix process_video_thumbnail (overwrite_output, robust
  duration probe across stream/format, eager frame load + temp cleanup)
  so videos stop ending up as the gray placeholder.
- library.py: new /maintenance/* endpoints — thumbnail-stats,
  regenerate-thumbnails (with media_type / only_failed filters), and a
  manual data-integrity cleanup trigger.
- Frontend Settings panel (gear in TopBar) surfacing those endpoints
  plus a re-scan button and live thumbnail status counts.
- PhotoThumbnail: stretch the auto-retry schedule for slow RAW jobs.

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

frontend/src/components/timeline/PhotoThumbnail.tsx

@@ -9,8 +9,10 @@ import { usePhotoStore } from '../../store/photoStore'
 export const PHOTO_DRAG_MIME = 'application/x-mulita-photos'
 
 // Auto-retry schedule (ms). Backend generates thumbs on-demand via Celery, so
-// first hit often 404s. Try a few times with backoff before giving up.
-const AUTO_RETRY_DELAYS = [1500, 3500, 6000]
+// the first hit on a freshly-scanned library returns 404 "not ready" until
+// the worker catches up. RAW postprocess can take several seconds per file
+// when the queue is deep, so the tail of the schedule is generous.
+const AUTO_RETRY_DELAYS = [1500, 3500, 7000, 12000, 20000]
 
 interface PhotoThumbnailProps {
   photo: Photo