fix(artwork): address blurhash review findings

- force recompute on image-cache miss, so in-place cover/sidecar swaps and
  agent image updates refresh the stored hash even when no entity row moved
- exclude external_info_updated_at from the artwork version: agent TTL
  refreshes bump it with an unchanged image, churning Finamp's cover cache
- lazy-start the worker goroutine and bound each computation with a 30s
  timeout; remember no-result entities per version to avoid re-decoding
  placeholders on every serve
- error (instead of silently skipping) on unknown artwork kinds in persist

model/album.go

@@ -77,16 +77,14 @@ func (a Album) CoverArtID() ArtworkID {
 	return artworkIDFromAlbum(a)
 }
 
-// ArtworkUpdatedAt is the album's artwork version: the newest row timestamp that can affect
-// which cover image is served (scan updates, imports, agent-fetched external images).
+// ArtworkUpdatedAt is the album's artwork version. ExternalInfoUpdatedAt is deliberately excluded:
+// it bumps on every agent TTL refresh even when the image is unchanged, and actual image changes
+// are caught by the image-cache-miss recompute instead.
 func (a Album) ArtworkUpdatedAt() time.Time {
 	t := a.UpdatedAt
 	if a.ImportedAt.After(t) {
 		t = a.ImportedAt
 	}
-	if a.ExternalInfoUpdatedAt != nil && a.ExternalInfoUpdatedAt.After(t) {
-		t = *a.ExternalInfoUpdatedAt
-	}
 	return t
 }
 

model/album_test.go

@@ -65,8 +65,8 @@ var _ = Describe("Album.ArtworkUpdatedAt", func() {
 		al := Album{UpdatedAt: base, ImportedAt: later}
 		Expect(al.ArtworkUpdatedAt()).To(Equal(later))
 	})
-	It("returns ExternalInfoUpdatedAt when it is the newest", func() {
+	It("ignores ExternalInfoUpdatedAt (agent TTL refreshes bump it without an image change)", func() {
 		al := Album{UpdatedAt: base, ImportedAt: later, ExternalInfoUpdatedAt: &latest}
-		Expect(al.ArtworkUpdatedAt()).To(Equal(latest))
+		Expect(al.ArtworkUpdatedAt()).To(Equal(later))
 	})
 })

model/artist.go

@@ -66,17 +66,14 @@ func (a Artist) CoverArtID() ArtworkID {
 	return artworkIDFromArtist(a)
 }
 
-// ArtworkUpdatedAt is the artist's artwork version; images often arrive via external agents,
-// which bump ExternalInfoUpdatedAt rather than UpdatedAt.
+// ArtworkUpdatedAt is the artist's artwork version. ExternalInfoUpdatedAt is deliberately
+// excluded: it bumps on every agent TTL refresh even when the image is unchanged, and actual
+// image changes are caught by the image-cache-miss recompute instead.
 func (a Artist) ArtworkUpdatedAt() time.Time {
-	var t time.Time
-	if a.UpdatedAt != nil {
-		t = *a.UpdatedAt
+	if a.UpdatedAt == nil {
+		return time.Time{}
 	}
-	if a.ExternalInfoUpdatedAt != nil && a.ExternalInfoUpdatedAt.After(t) {
-		t = *a.ExternalInfoUpdatedAt
-	}
-	return t
+	return *a.UpdatedAt
 }
 
 func (a Artist) UploadedImagePath() string {

model/artist_test.go

@@ -37,10 +37,10 @@ var _ = Describe("Artist.ArtworkUpdatedAt", func() {
 	It("handles nil timestamps", func() {
 		Expect(model.Artist{}.ArtworkUpdatedAt()).To(Equal(time.Time{}))
 	})
-	It("returns UpdatedAt when newest", func() {
+	It("returns UpdatedAt", func() {
 		Expect(model.Artist{UpdatedAt: &later, ExternalInfoUpdatedAt: &base}.ArtworkUpdatedAt()).To(Equal(later))
 	})
-	It("returns ExternalInfoUpdatedAt when newest", func() {
-		Expect(model.Artist{UpdatedAt: &base, ExternalInfoUpdatedAt: &later}.ArtworkUpdatedAt()).To(Equal(later))
+	It("ignores ExternalInfoUpdatedAt (agent TTL refreshes bump it without an image change)", func() {
+		Expect(model.Artist{UpdatedAt: &base, ExternalInfoUpdatedAt: &later}.ArtworkUpdatedAt()).To(Equal(base))
 	})
 })