Add artwork_url to music track summaries

Extended the music track summary object in `MusicRoutes` to include the `artwork_url` field. This provides front-end clients with direct access to track artwork, improving UI and metadata presentation.

src/main/java/me/bobert/netneutrality/http/server/MusicRoutes.java

@@ -474,6 +474,7 @@ public final class MusicRoutes {
         summary.put("author", track.getInfo().getAuthor());
         summary.put("uri", track.getInfo().getUri());
         summary.put("duration_ms", track.getInfo().getLength());
+        summary.put("artwork_url", track.getInfo().getArtworkUrl());
         return summary;
     }