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).
Write falls through to a real write when the liveness touch fails, and sweep retention now matches the recorded mime's extension so obsolete variants are reclaimed.
Duplicate ImageStore writes refresh the file mtime and Remove skips files newer than the cutoff, so overlapping acquisitions cannot lose their store files to a concurrent prune.
Sweep now honors an mtime grace window (in-flight acquisitions and temp files), reacquired orphans reset the prune grace window, and the queue mock implements real stale-absent semantics.
