- 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>
