Add per-user local playlists: Playlist + PlaylistItem models (with forward-looking
kind/source/yt_playlist_id/dirty columns for the later YouTube-sync phases) and
migration 0011. New /api/playlists routes: list (with optional contains=<video_id>
membership flags for the add-to-playlist popover), create, get detail (items
serialized via the feed serializer, with per-user watch state), rename, delete,
add/remove item, and reorder. Watch later (built-in) is protected from deletion.
