The hydration fallback assigned a found album hash to every fallback track, but a
multi-disc track's CoverArtID emits a dc- id served from disc-specific art whose hash
is unknown at hydration time. Advertising dc-..._<albumHash> gave clients a content-
version that never changes when the disc image does, breaking id-based refresh. Only
stamp the album hash for single-disc tracks (DiscNumber == 0); multi-disc tracks stay
unhashed and rely on the correct ETag returned by the served response.
Round-1's hydration fix still copied the album's known-absent onto a non-eligible
(or own-absent) track, but MediaFile.CoverArtID routes a multi-disc track to disc
art, which resolves provisionally and is never known-absent. Marking it absent made
Subsonic omit coverArt so clients never requested a valid disc image. Only mark a
single-disc track absent, and only when its own art won't resolve.
An embedded-eligible track with no resolved item_artwork row inherited the album's
ImageAbsent, so when the album resolved absent (e.g. CoverArtPriority without
'embedded') the track's coverArt was omitted permanently — the client never
requested it, so the lazy mediafile path never resolved it — even though the
serving path would extract and serve the track's own embedded art. Hydration now
never copies the album's absence onto an eligible-but-unresolved track.
