The provisional read-through and dangling re-enqueue used Enqueue, whose upsert
resets retry_at, so any browse of an unresolved entity that was backing off after
an external failure made it immediately eligible again — defeating the exponential
backoff during a provider outage. Add EnqueueBump, which raises priority but leaves
an existing row's retry_at intact, and route the serving path through it. Scan and
manual re-resolve keep Enqueue's reset (a detected change wants immediate retry).
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).
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.
