fix(artwork): backfill warm caches and supersede pending gone on refill

Two issues in the fill-triggered blurhash recompute:

1. Enqueuing was gated on a cache miss (!r.Cached). On an upgraded instance the
   image cache is persisted and adopted across restarts, so already-cached
   artwork serves as a cache hit and never enqueued, leaving its migrated-empty
   blur_hash as a synthetic value indefinitely. Enqueue on every original-size
   serve instead: the worker's freshness guard turns already-hashed rows into a
   cheap read and only recomputes when the hash is stale or missing.

2. Enqueue merged the gone flag with a sticky OR, so a cover restored right
   after a missing-art serve kept gone=true and took the clear-and-return path,
   never recomputing. A successful serve proves the artwork exists, so it now
   clears any pending gone for that artwork; a gone that follows a fill still
   sticks.

core/artwork/artwork.go

@@ -87,10 +87,11 @@ func (a *artwork) Get(ctx context.Context, artID model.ArtworkID, size int, squa
 		}
 		return nil, time.Time{}, err
 	}
-	if a.blurHashes != nil && size == 0 && !square && !r.Cached {
-		// An original-size cache fill is exactly when the served bytes change: the single recompute
-		// trigger. Resized fills recurse through Get(size=0); a disabled cache reports every serve as
-		// a fill (the worker's unchanged-hash guard keeps that write-free).
+	if a.blurHashes != nil && size == 0 && !square {
+		// Every original-size serve carries the reader's true version; the worker's freshness guard
+		// turns already-hashed rows into a cheap read and only recomputes when the hash is stale or
+		// missing. Enqueuing on cache hits too (not just fills) is what backfills warm caches adopted
+		// from a pre-blurhash version, whose rows migrated in with an empty hash.
 		a.blurHashes.Enqueue(artID, artReader.LastUpdated())
 	}
 	return r, artReader.LastUpdated(), nil

core/artwork/blurhash_updater.go

@@ -83,10 +83,16 @@ func (u *blurHashUpdater) enqueue(artID model.ArtworkID, req enqueueRequest) {
 		go u.run(ctx)
 	}
 	prev := u.buffer[artID]
-	if req.snapshot.After(prev.snapshot) {
-		prev.snapshot = req.snapshot
+	if !req.snapshot.IsZero() {
+		// A successful serve proves the artwork exists, so it supersedes any pending gone request for
+		// the same artwork (a cover restored right after a missing-art serve must still recompute).
+		prev.gone = false
+		if req.snapshot.After(prev.snapshot) {
+			prev.snapshot = req.snapshot
+		}
+	} else if req.gone {
+		prev.gone = true
 	}
-	prev.gone = prev.gone || req.gone
 	u.buffer[artID] = prev
 	u.mutex.Unlock()
 	select {

core/artwork/blurhash_updater_internal_test.go

@@ -45,7 +45,7 @@ var _ = Describe("blurHashUpdater", func() {
 			Expect(u.buffer[id].gone).To(BeFalse())
 		})
 
-		It("merges a gone flag onto a pending snapshot for the same artwork", func() {
+		It("keeps a gone flag when it follows a pending fill (cover then vanished)", func() {
 			id := model.Album{ID: "al-1"}.CoverArtID()
 			t1 := time.Date(2024, 1, 1, 0, 0, 0, 0, time.UTC)
 			u.Enqueue(id, t1)
@@ -54,6 +54,15 @@ var _ = Describe("blurHashUpdater", func() {
 			Expect(u.buffer[id].gone).To(BeTrue())
 		})
 
+		It("lets a successful fill supersede a pending gone (cover restored)", func() {
+			id := model.Album{ID: "al-1"}.CoverArtID()
+			t1 := time.Date(2024, 1, 1, 0, 0, 0, 0, time.UTC)
+			u.EnqueueGone(id)
+			u.Enqueue(id, t1)
+			Expect(u.buffer[id].snapshot).To(Equal(t1))
+			Expect(u.buffer[id].gone).To(BeFalse())
+		})
+
 		It("ignores other artwork kinds", func() {
 			u.Enqueue(model.ArtworkID{Kind: model.KindMediaFileArtwork, ID: "mf-1"}, time.Time{})
 			u.EnqueueGone(model.ArtworkID{Kind: model.KindRadioArtwork, ID: "ra-1"})