fix(album): don't wipe covers on album merges; bust disc/song art caches

Adversarial review findings:

- CopyAttributes copied uploaded_image/cover_art_updated_at unconditionally,
  so merging an album without a cover into one with a cover (retag onto an
  existing album ID, or phase-2 moved-track matching) blanked the
  destination's uploaded cover. Skip empty/NULL source values, mirroring the
  created_at guard, and make the mock faithful to the same semantics.

- The disc and mediafile artwork readers did not fold cover_art_updated_at
  into their cache keys, so song/disc art reached through the fromAlbum
  fallback kept serving stale bytes after a cover upload or delete.

- The UI cover cache-buster compared timestamps lexicographically; mixed
  offset/fractional formats could pick the older one. Compare as dates.

core/artwork/reader_disc.go

@@ -110,6 +110,9 @@ func newDiscArtworkReader(ctx context.Context, a *artwork, artID model.ArtworkID
 	if imagesUpdatedAt != nil {
 		r.cacheKey.lastUpdate = utils.TimeNewest(r.cacheKey.lastUpdate, *imagesUpdatedAt)
 	}
+	if al.CoverArtUpdatedAt != nil {
+		r.cacheKey.lastUpdate = utils.TimeNewest(r.cacheKey.lastUpdate, *al.CoverArtUpdatedAt)
+	}
 	return r, nil
 }
 

core/artwork/reader_mediafile.go

@@ -49,6 +49,9 @@ func newMediafileArtworkReader(ctx context.Context, artwork *artwork, artID mode
 	if imagesUpdatedAt != nil && imagesUpdatedAt.After(a.cacheKey.lastUpdate) {
 		a.cacheKey.lastUpdate = *imagesUpdatedAt
 	}
+	if al.CoverArtUpdatedAt != nil && al.CoverArtUpdatedAt.After(a.cacheKey.lastUpdate) {
+		a.cacheKey.lastUpdate = *al.CoverArtUpdatedAt
+	}
 	return a, nil
 }
 

persistence/album_repository.go

@@ -287,6 +287,10 @@ func (r *albumRepository) CopyAttributes(fromID, toID string, columns ...string)
 		if col == "created_at" && (!v.Valid || v.String == "" || strings.HasPrefix(v.String, "0001-")) {
 			continue
 		}
+		// A source without an uploaded cover must not wipe one the destination has.
+		if (col == "uploaded_image" || col == "cover_art_updated_at") && (!v.Valid || v.String == "") {
+			continue
+		}
 		to[col] = v
 	}
 	if len(to) == 0 {

persistence/album_repository_test.go

@@ -116,6 +116,14 @@ var _ = Describe("AlbumRepository", func() {
 			Expect(err).ToNot(HaveOccurred())
 			Expect(got.UploadedImage).To(Equal("copy-src_cover.jpg"))
 		})
+		It("does not wipe the destination's cover when the source has none", func() {
+			Expect(albumRepo.UpdateImage("copy-dst", "copy-dst_cover.jpg")).To(Succeed())
+			Expect(albumRepo.CopyAttributes("copy-src", "copy-dst", "uploaded_image", "cover_art_updated_at")).To(Succeed())
+			got, err := albumRepo.Get("copy-dst")
+			Expect(err).ToNot(HaveOccurred())
+			Expect(got.UploadedImage).To(Equal("copy-dst_cover.jpg"))
+			Expect(got.CoverArtUpdatedAt).ToNot(BeNil())
+		})
 	})
 
 	Describe("GetCursor", func() {

tests/mock_album_repo.go

@@ -190,9 +190,14 @@ func (m *MockAlbumRepo) CopyAttributes(fromID, toID string, columns ...string) e
 		case "created_at":
 			to.CreatedAt = from.CreatedAt
 		case "uploaded_image":
-			to.UploadedImage = from.UploadedImage
+			// Mirrors the real repo: an empty source never wipes the destination's cover
+			if from.UploadedImage != "" {
+				to.UploadedImage = from.UploadedImage
+			}
 		case "cover_art_updated_at":
-			to.CoverArtUpdatedAt = from.CoverArtUpdatedAt
+			if from.CoverArtUpdatedAt != nil {
+				to.CoverArtUpdatedAt = from.CoverArtUpdatedAt
+			}
 		}
 	}
 	if m.CopyAttributesCalls == nil {

ui/src/subsonic/index.js

@@ -82,10 +82,10 @@ const getAvatarUrl = (username, size) =>
 
 const getCoverArtUrl = (record, size, square) => {
   // Bust the cache on the newest of updatedAt / coverArtUpdatedAt (album cover
-  // uploads bump the latter) — ISO timestamps sort chronologically.
+  // uploads bump the latter).
   const cacheKey = [record.updatedAt, record.coverArtUpdatedAt]
     .filter(Boolean)
-    .sort()
+    .sort((a, b) => new Date(a) - new Date(b))
     .pop()
   const options = {
     ...(cacheKey && { _: cacheKey }),