feat(artwork): advertise album art for tracks whose embedded cover is the album's

Most albums embed the same picture in every track, so clients requesting
per-track cover art (mf- ids) download and cache the same image once per
song. This change hashes the embedded picture at scan time (XXH3 of the
bytes, the same digest the artwork pipeline uses) and stores it on
media_file.embed_art_hash. ToAlbum stores the hash of the track it already
picks as the album's embedded cover (EmbedArtPath) on album.embed_art_hash.

A new MediaFile.HasOwnCoverArt replaces the scattered
"HasCoverArt && EnableMediaFileCoverArt" checks: a track whose hash equals
its album's now returns the disc/album artwork id from CoverArtID, so the
Subsonic, Jellyfin and native responses point every matching song at one
cached image. Serving a stale mf- URL follows the same rule, so what is
advertised and what is served agree. Tracks with a different embedded
picture (compilations, per-track art) keep their own ids.

Rows scanned before this change have an empty hash and keep the previous
behavior until a full scan; no config option is added. The album hash is
hydrated onto tracks with one batched PK lookup per page, only when the
page has hashed tracks. BestImageIndex is exported from core/artwork so the
scanner hashes the same picture the resolver serves.

model/metadata/map_mediafile.go

@@ -65,6 +65,7 @@ func (md Metadata) ToMediaFile(libID int, folderID string) model.MediaFile {
 
 	// General properties
 	mf.HasCoverArt = md.HasPicture()
+	mf.EmbedArtHash = md.PictureHash()
 	mf.Duration = md.Length()
 	mf.BitRate = md.AudioProperties().BitRate
 	mf.SampleRate = md.AudioProperties().SampleRate

model/metadata/metadata.go

@@ -23,6 +23,7 @@ type Info struct {
 	Tags            model.RawTags
 	AudioProperties AudioProperties
 	HasPicture      bool
+	PictureHash     string
 }
 
 type FileInfo interface {
@@ -69,20 +70,22 @@ func NewPair(key, value string) string {
 
 func New(filePath string, info Info) Metadata {
 	return Metadata{
-		filePath:   filePath,
-		fileInfo:   info.FileInfo,
-		tags:       clean(filePath, info.Tags),
-		audioProps: info.AudioProperties,
-		hasPicture: info.HasPicture,
+		filePath:    filePath,
+		fileInfo:    info.FileInfo,
+		tags:        clean(filePath, info.Tags),
+		audioProps:  info.AudioProperties,
+		hasPicture:  info.HasPicture,
+		pictureHash: info.PictureHash,
 	}
 }
 
 type Metadata struct {
-	filePath   string
-	fileInfo   FileInfo
-	tags       model.Tags
-	audioProps AudioProperties
-	hasPicture bool
+	filePath    string
+	fileInfo    FileInfo
+	tags        model.Tags
+	audioProps  AudioProperties
+	hasPicture  bool
+	pictureHash string
 }
 
 func (md Metadata) FilePath() string     { return md.filePath }
@@ -95,6 +98,7 @@ func (md Metadata) Suffix() string {
 func (md Metadata) AudioProperties() AudioProperties         { return md.audioProps }
 func (md Metadata) Length() float32                          { return float32(md.audioProps.Duration.Milliseconds()) / 1000 }
 func (md Metadata) HasPicture() bool                         { return md.hasPicture }
+func (md Metadata) PictureHash() string                      { return md.pictureHash }
 func (md Metadata) All() model.Tags                          { return md.tags }
 func (md Metadata) Strings(key model.TagName) []string       { return md.tags[key] }
 func (md Metadata) String(key model.TagName) string          { return md.first(key) }

model/metadata/metadata_test.go

@@ -36,8 +36,9 @@ var _ = Describe("Metadata", func() {
 				Duration: time.Minute * 3,
 				BitRate:  320,
 			},
-			HasPicture: true,
-			FileInfo:   testFileInfo{fileInfo},
+			HasPicture:  true,
+			PictureHash: "0123456789abcdef",
+			FileInfo:    testFileInfo{fileInfo},
 		}
 	})
 
@@ -60,6 +61,7 @@ var _ = Describe("Metadata", func() {
 				Expect(md.AudioProperties()).To(Equal(props.AudioProperties))
 				Expect(md.Length()).To(Equal(float32(3 * 60)))
 				Expect(md.HasPicture()).To(Equal(props.HasPicture))
+				Expect(md.PictureHash()).To(Equal(props.PictureHash))
 				Expect(md.Strings(model.TagTrackArtist)).To(Equal([]string{"First Artist", "Second Artist"}))
 				Expect(md.String(model.TagTrackArtist)).To(Equal("First Artist"))
 				Expect(md.Int(model.TagCatalogNumber)).To(Equal(int64(1234)))