fix(ui): bust song artwork URLs when the album cover changes

Song art URLs (mf- ids: player bar, queue) were cache-busted only by the
song's own updatedAt, and cover responses carry a 10-year max-age — so after
an album cover upload/delete the browser kept serving the stale cached image
(including the placeholder, for previously artless albums) until the file
itself was rescanned.

Expose the album's cover_art_updated_at on song payloads via a scalar
subquery (a join would make unqualified references ambiguous — media_file
and album share column names), in both the media_file select and the
playlist tracks query, and pass it through the player queue builder so
getCoverArtUrl picks it up.

model/mediafile.go

@@ -30,11 +30,14 @@ type MediaFile struct {
 	LibraryID   int    `structs:"library_id" json:"libraryId" hash:"ignore"`
 	LibraryPath string `structs:"-" json:"libraryPath" hash:"ignore"`
 	LibraryName string `structs:"-" json:"libraryName" hash:"ignore"`
-	FolderID    string `structs:"folder_id" json:"folderId" hash:"ignore"`
-	Path        string `structs:"path" json:"path" hash:"ignore"`
-	Title       string `structs:"title" json:"title"`
-	Album       string `structs:"album" json:"album"`
-	ArtistID    string `structs:"artist_id" json:"artistId"` // Deprecated: Use Participants instead
+	// CoverArtUpdatedAt mirrors the album's manual-cover timestamp so clients can
+	// bust song artwork URLs that resolve to the album cover.
+	CoverArtUpdatedAt *time.Time `structs:"-" json:"coverArtUpdatedAt,omitempty" hash:"ignore"`
+	FolderID          string     `structs:"folder_id" json:"folderId" hash:"ignore"`
+	Path              string     `structs:"path" json:"path" hash:"ignore"`
+	Title             string     `structs:"title" json:"title"`
+	Album             string     `structs:"album" json:"album"`
+	ArtistID          string     `structs:"artist_id" json:"artistId"` // Deprecated: Use Participants instead
 	// Artist is the display name used for the artist.
 	Artist        string `structs:"artist" json:"artist"`
 	AlbumArtistID string `structs:"album_artist_id" json:"albumArtistId"` // Deprecated: Use Participants instead

persistence/mediafile_repository.go

@@ -184,7 +184,10 @@ func (r *mediaFileRepository) UpdateProbeData(id string, data string) error {
 }
 
 func (r *mediaFileRepository) selectMediaFile(options ...model.QueryOptions) SelectBuilder {
-	sql := r.newSelect(options...).Columns("media_file.*", "library.path as library_path", "library.name as library_name").
+	// Scalar subquery (not a join): album shares column names with media_file, and a join
+	// would make unqualified filter/sort references ambiguous.
+	sql := r.newSelect(options...).Columns("media_file.*", "library.path as library_path", "library.name as library_name",
+		"(select cover_art_updated_at from album where album.id = media_file.album_id) as cover_art_updated_at").
 		LeftJoin("library on media_file.library_id = library.id")
 	sql = r.withAnnotation(sql, "media_file.id")
 	sql = r.withBookmark(sql, "media_file.id")

persistence/mediafile_repository_test.go

@@ -29,6 +29,27 @@ var _ = Describe("MediaRepository", func() {
 		mr = NewMediaFileRepository(ctx, GetDBXBuilder())
 	})
 
+	Describe("CoverArtUpdatedAt", func() {
+		It("exposes the album's cover timestamp on its songs", func() {
+			albumRepo := NewAlbumRepository(request.WithUser(GinkgoT().Context(), model.User{ID: "userid"}), GetDBXBuilder())
+			Expect(albumRepo.Put(&model.Album{ID: "cover-al", Name: "cover", LibraryID: 1})).To(Succeed())
+			Expect(mr.Put(&model.MediaFile{ID: "cover-mf", LibraryID: 1, AlbumID: "cover-al", Path: "test/cover-mf.mp3"})).To(Succeed())
+			DeferCleanup(func() {
+				_, _ = mr.(*mediaFileRepository).executeSQL(squirrel.Delete("media_file").Where(squirrel.Eq{"id": "cover-mf"}))
+				_, _ = mr.(*mediaFileRepository).executeSQL(squirrel.Delete("album").Where(squirrel.Eq{"id": "cover-al"}))
+			})
+
+			got, err := mr.Get("cover-mf")
+			Expect(err).ToNot(HaveOccurred())
+			Expect(got.CoverArtUpdatedAt).To(BeNil())
+
+			Expect(albumRepo.UpdateImage("cover-al", "cover-al_x.jpg")).To(Succeed())
+			got, err = mr.Get("cover-mf")
+			Expect(err).ToNot(HaveOccurred())
+			Expect(got.CoverArtUpdatedAt).ToNot(BeNil())
+		})
+	})
+
 	Describe("GetCursor", func() {
 		It("yields the same media files as GetAll", func() {
 			opts := model.QueryOptions{Sort: "title"}

persistence/playlist_repository.go

@@ -314,6 +314,7 @@ func (r *playlistRepository) tracksQuery(query SelectBuilder, id string) SelectB
 			"playlist_tracks.*",
 			"library.path as library_path",
 			"library.name as library_name",
+			"(select cover_art_updated_at from album where album.id = f.album_id) as cover_art_updated_at",
 		).
 		LeftJoin("annotation on (" +
 			"annotation.item_id = media_file_id" +

ui/src/reducers/playerReducer.js

@@ -91,6 +91,7 @@ const mapToAudioLists = (item) => {
       {
         id: trackId,
         updatedAt: item.updatedAt,
+        coverArtUpdatedAt: item.coverArtUpdatedAt,
         album: item.album,
       },
       300,