Local 'user' playlists (not Watch later, not YouTube mirrors) can now be pushed
to YouTube: create a new playlist on first push, else reconcile membership + order
so YouTube matches local (local wins). plan_push computes a dry-run estimate
(inserts/deletes/reorders + quota units + divergence) read from the live YouTube
state; push refuses if the estimate exceeds the remaining daily quota so a big
playlist can't strand half-pushed. Reorder uses an insertion-sort move count that
matches what executes. Edits to a linked playlist set dirty; a successful push
clears it. DELETE accepts on_youtube to also delete the playlist on YouTube. The
read-sync now skips YouTube playlists already owned by a local export, avoiding
duplicate read-only mirrors.
Mirror each user's own YouTube playlists into local source='youtube' playlists,
one-way (YT -> local). New client methods iter_my_playlists / iter_my_playlist_video_ids
(OAuth, so private playlists work); sync/playlists.py reconciles the mirror (matching YT
order) and ingests any playlist videos not in the shared catalog yet (with stub channels).
POST /api/playlists/sync-youtube for manual sync (read-scope gated, per-user quota) plus a
scheduler job (playlist_sync_minutes, default 6h) that syncs all read-scope users. YouTube's
Watch Later / History are not API-accessible and are never synced.
