playlists are followable but had no scan. new 'Scan Watchlist Playlists'
automation, sibling of the channel scan but a different rule: a playlist is a
curated finite set, so MIRROR it — wishlist every long-form video you don't have
plus any later additions (no forward-looking baseline, no last-N net).
playlist-as-show: videos wishlisted under the playlist's title, so the worker
files them as 'Playlist Name / Season YEAR / ... - date - title' (the ytdl-sub
tv_show_name-on-a-playlist convention). reuses all the channel plumbing — same
wishlist rows, same Download YouTube Wishlist drain, quality + org template.
seeded every 6h (Auto-Scan Watchlist Playlists); block + registration + icon +
drift test. dedup reuses wishlisted_video_ids_for_channel (parent_source_id=PL).
seam-tested; 944-test sweep green.
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