feat: add disc number and compilation metadata tags

- disc tag written as discNum/discTotal (e.g. 2/2) for multi-disc albums
- compilation=1 set when iTunes collectionType is Compilation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/novasonic-server.js

@@ -108,7 +108,7 @@ async function downloadArtwork(url, dir) {
 }
 
 // Download a single track: yt-dlp audio + ffmpeg clean metadata + iTunes cover embedded
-async function downloadTrack({ jobId, artist, albumArtist, title, album, year, trackNum, totalTracks, genre, artworkUrl, outputDir }) {
+async function downloadTrack({ jobId, artist, albumArtist, title, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, outputDir }) {
   const mainArtist = albumArtist || artist;
   const safeArtist = sanitize(mainArtist);
   const safeAlbum  = sanitize(album || 'Singles');
@@ -197,6 +197,8 @@ async function downloadTrack({ jobId, artist, albumArtist, title, album, year, t
     if (trackNum)    ffArgs.push('-metadata', `track=${totalTracks ? `${trackNum}/${totalTracks}` : String(trackNum)}`);
     if (totalTracks) ffArgs.push('-metadata', `totaltracks=${String(totalTracks)}`);
     if (genre)         ffArgs.push('-metadata', `genre=${genre}`);
+    if (discNum)       ffArgs.push('-metadata', `disc=${discTotal ? `${discNum}/${discTotal}` : String(discNum)}`);
+    if (compilation)   ffArgs.push('-metadata', `compilation=1`);
     if (hasCover) {
       ffArgs.push(
         '-metadata:s:v', 'title=Album cover',
@@ -242,6 +244,9 @@ async function downloadAlbumTracks({ jobId, artist, albumTitle, tracks, artworkU
         year:        track.releaseDate ? new Date(track.releaseDate).getFullYear() : undefined,
         trackNum:    track.trackNumber     || undefined,
         totalTracks: total,
+        discNum:     track.discNumber      || undefined,
+        discTotal:   track.discCount       || undefined,
+        compilation: track.collectionType === 'Compilation' || undefined,
         genre:       track.primaryGenreName || undefined,
         artworkUrl:  artworkUrl || track.artworkUrl100,
         outputDir:   CONFIG.musicPath,