Two worker/enqueue fixes from review:
- playlistRepository.Put assigned the generated id to the caller's Playlist but passed
the stale copy (empty id) to refreshCounters, enqueueing a pl|"" row the worker
failed until the daily dangling purge while the real playlist went unresolved. Set
the id on the copy before enqueueing.
- The drain refresh batch only included found/foundStale, so a cover removed by a scan
(found -> absent) never notified clients, leaving the old immutable image displayed.
Broadcast absent outcomes too; precache still only warms found/foundStale.
The artwork worker's drain speed was governed by two hidden Dev flags,
DevArtworkWorkerConcurrency and DevArtworkExternalRPS, both defaulting to 2.
On a large library's one-time backfill the external rate limiter is the real
ceiling: every art-less item waits on it before the (rate-limited) external
lookup, so the drain crawls at ~RPS items/sec while local-art items are
unaffected.
Promote both to documented, supported options: ArtworkWorkerConcurrency
(default 4) sets local-resolution parallelism, ArtworkExternalMaxRPS
(default 2, 0 = unlimited) caps external-agent lookups to stay polite to
Last.fm/Deezer/etc. Operators can now trade first-backfill speed against
external-API rate limits. The old Dev names still map for backward compat.
With CoverArtPriority="external,cover.jpg", a transient external failure
followed by a folder hit dropped the external error: the worker recorded
found and deleted the queue row, so the configured higher-priority external
art was never retried. Carry extError onto the fallback resolution and add
an outcomeFoundStale that persists+serves the art but reschedules via
MarkFailed, giving the external source another chance. When external later
answers definitively-not-found, the hit is not stale and the row is deleted.
Queue rows for deleted entities failed forever (Get -> ErrNotFound -> failed -> capped retries, unbounded). Add ArtworkQueueRepository.PurgeDangling, called from Prune next to the item_artwork purge. Separately, the found/absent path unconditionally deleted the dequeued row, erasing a concurrent scan re-enqueue; switch to DeleteIfUnchanged, which deletes only while retry_at still matches the dequeued value (verified retry_at is the column an Enqueue upsert resets).
