feat: extract embedded album artwork during scans

Add a dedicated artwork cache directory and save embedded album art from audio tags during library scans. Prefer embedded artwork for album cover resolution while keeping sidecar image files as the fallback path.

.env.example

@@ -2,6 +2,7 @@ APP_ENV=development
 SERVER_HOST=0.0.0.0
 SERVER_PORT=4040
 DATABASE_PATH=./data/app.db
+ARTWORK_CACHE_DIR=./data/artwork
 MEDIA_ROOT=./media
 CORS_ORIGINS=http://localhost:5173
 DEFAULT_ADMIN_USERNAME=demo

cmd/server/main.go

@@ -33,7 +33,7 @@ func main() {
 		log.Fatalf("database seed failed: %v", err)
 	}
 
-	scanService := scanner.NewService(database, cfg.MediaRoot)
+	scanService := scanner.NewService(database, cfg.MediaRoot, cfg.ArtworkCacheDir)
 	if scanService.HasMediaFiles() {
 		if result, err := scanService.Scan(ctx); err != nil {
 			log.Printf("startup scan failed: %v", err)

internal/config/config.go

@@ -7,6 +7,7 @@ type Config struct {
 	ServerHost   string
 	ServerPort   string
 	DatabasePath string
+	ArtworkCacheDir string
 	MediaRoot    string
 	CORSOrigins  string
 	DefaultAdminUsername string
@@ -19,6 +20,7 @@ func Load() Config {
 		ServerHost:   getenv("SERVER_HOST", "0.0.0.0"),
 		ServerPort:   getenv("SERVER_PORT", "4040"),
 		DatabasePath: getenv("DATABASE_PATH", "./data/app.db"),
+		ArtworkCacheDir: getenv("ARTWORK_CACHE_DIR", "./data/artwork"),
 		MediaRoot:    getenv("MEDIA_ROOT", "./media"),
 		CORSOrigins:  getenv("CORS_ORIGINS", "http://localhost:5173"),
 		DefaultAdminUsername: getenv("DEFAULT_ADMIN_USERNAME", "demo"),

internal/scanner/service.go

@@ -38,6 +38,7 @@ type Result struct {
 type Service struct {
 	db        *sql.DB
 	mediaRoot string
+	artworkRoot string
 	mu        sync.RWMutex
 	status    Status
 }
@@ -78,8 +79,8 @@ type scannedTrack struct {
 	ContentType  string
 }
 
-func NewService(db *sql.DB, mediaRoot string) *Service {
+func NewService(db *sql.DB, mediaRoot, artworkRoot string) *Service {
-	return &Service{db: db, mediaRoot: mediaRoot}
+	return &Service{db: db, mediaRoot: mediaRoot, artworkRoot: artworkRoot}
 }
 
 func (s *Service) HasMediaFiles() bool {
@@ -327,7 +328,13 @@ func (s *Service) scanFile(path string) (scannedItem, error) {
 	artistID := hashID("artist", artistName)
 	albumID := hashID("album", artistName, albumTitle, fmt.Sprintf("%d", year))
 	trackID := hashID("track", path)
-	coverArt := findCoverArt(filepath.Dir(path))
+	coverArt := ""
+	if err == nil {
+		coverArt = s.extractEmbeddedCoverArt(metadata, albumID)
+	}
+	if coverArt == "" {
+		coverArt = findCoverArt(filepath.Dir(path))
+	}
 
 	return scannedItem{
 		artist: scannedArtist{
@@ -417,6 +424,44 @@ func findCoverArt(dir string) string {
 	return ""
 }
 
+func (s *Service) extractEmbeddedCoverArt(metadata tag.Metadata, albumID string) string {
+	if metadata == nil {
+		return ""
+	}
+
+	picture := metadata.Picture()
+	if picture == nil || len(picture.Data) == 0 {
+		return ""
+	}
+
+	if s.artworkRoot == "" {
+		return ""
+	}
+
+	if err := os.MkdirAll(s.artworkRoot, 0o755); err != nil {
+		return ""
+	}
+
+	filename := albumID + coverExtension(picture.MIMEType)
+	path := filepath.Join(s.artworkRoot, filename)
+	if err := os.WriteFile(path, picture.Data, 0o644); err != nil {
+		return ""
+	}
+
+	return path
+}
+
+func coverExtension(mimeType string) string {
+	switch strings.ToLower(mimeType) {
+	case "image/png":
+		return ".png"
+	case "image/webp":
+		return ".webp"
+	default:
+		return ".jpg"
+	}
+}
+
 func (s *Service) tryMarkStarted() bool {
 	s.mu.Lock()
 	defer s.mu.Unlock()