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.
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