Adds a VideoBackfillWorker base that enriches already-identified items BY id
(vs the matcher workers), with the exact same lifecycle + get_stats() shape so
the engine registry, /api/video/enrichment routes, and Manage-Workers modal
drive them identically.
Workers:
- fanart.tv (free key): gap-fills logo/clearart/banner/backdrop/poster art
- OpenSubtitles (free key): records subtitle-language availability per title
- Return YouTube Dislike (no key): like/dislike estimates on cached videos
- SponsorBlock (no key): crowd segments per video
DB: youtube_video_stats + youtube_video_segments tables; fanart/subs columns;
backfill_next/mark/breakdown + youtube_enrich_* helpers; likes/dislikes merged
into get_channel_videos. Seam tests in tests/test_video_backfill.py.
