Two serving-correctness fixes from review:
- serveMediaFile served a persisted mf embedded image even after EnableMediaFileCoverArt
was turned off (the setting isn't in the config fingerprint, so found rows aren't
reprocessed). Direct mf- URLs now honor the setting at serve time and fall back to
disc/album art.
- The file-backed staleness check compared whole-second mtimes, so a same-second content
replacement (two writes in one second, or timestamp-preserving tools) could serve
different bytes under the old hash + immutable policy. RefMtime is now unix-nanoseconds
(no schema change; int64 column), detecting sub-second changes where the filesystem
records them.
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).
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.
