A playlist is now a first-class watchlist type alongside channels:
- core: parse_playlist_id (URL/bare id; rejects mixes RD… + personal lists) +
resolve_playlist → {playlist_id, title, channel_title, video_count, thumbnail,
videos} in the CURATOR's order (partial set, not by year).
- db: add/remove/list/watch_state for kind='playlist' video_watchlist rows
(mirrors channels; same surrogate scheme, separate kind so they don't collide).
- api: /youtube/resolve now detects playlist links; /youtube/playlist/follow +
/unfollow; the existing /youtube/playlist/<id> is upgraded to also return the
playlist meta + following (serves the channel-page expansion AND the new detail
view from one route); /youtube/channels also returns followed playlists.
Validated live (Lex Fridman playlist resolves title+owner+videos). 190 tests green.
Frontend (search detect + result card + detail view + watchlist display) next.
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