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.

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"),