Root cause (from the reporter's app.log): a ListenBrainz weekly playlist syncs
through the in-memory youtube_playlist_states discovery machine. When that live
state is lost — a Docker restart, or the discovery process ending while the user
waits for the media-server scan — the DB discover-download snapshot survives but
the live state is gone. Every recovery action (Cancel/Reset/Delete) then hit
`key not in states` and returned 404 "YouTube playlist not found" (hence the
confusing "Youtube" on a ListenBrainz playlist), leaving the playlist permanently
wedged with no way to dismiss or re-sync. Works for the maintainer because a
single session with no restart keeps the live state alive.
Fix — these are cleanup ops, so "the thing is already gone" is SUCCESS, not 404:
- cancel_sync core (shared by YouTube + ListenBrainz + Tidal/Deezer/Qobuz/...) →
missing key returns idempotent success.
- reset_youtube_playlist / delete_youtube_playlist → same.
The playlist becomes recoverable: Cancel/Reset clears the dead state and the user
re-syncs fresh.
Tests: cancel_sync core (missing key = idempotent 200 not 404; present key still
cancels + clears the worker + reverts phase); endpoint-level idempotency for
cancel/reset/delete; updated the old test that locked the 404 wedge. 834 sync/
discovery tests green.
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