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.

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)