serveMediaFile delegated an absent/ineligible track straight to AlbumCoverArtID,
skipping the disc-specific lookup that MediaFile.CoverArtID (and the deleted legacy
reader) use. On multi-disc albums with per-disc images that served the album cover
instead of the configured disc artwork. Delegate through DiscCoverArtID.
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).
