Skip already cached songs on startup

src/lib/server/musicCache.ts

@@ -57,6 +57,7 @@ export function refreshMusicCache() {
 	(async () => {
 		await prepareCacheDirectory();
 		for (const videoId of playlistVideoIds) {
+			if (readySongs.has(videoId)) continue;
 			try {
 				await downloadSong(videoId);
 				console.log(`Music cached: ${videoId}`);