- New playlists router: fetch channel playlists from YouTube, index
playlist videos, browse by playlist with pagination
- Playlist model gets video_ids column to store ordered video list
- Register playlists router in main.py with DB migration
- Add Playlists tab to Channel page: grid of playlist cards, click to
browse videos, index/re-index per playlist
- Fix explore older videos skipping all entries without published_at;
flat-playlist entries for older videos rarely include timestamp data
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
