feat(api): deliver loudness gains to every client (M464 #4997)

ReplayGain 2.0 values (gain to -18 LUFS, linear peak) derived from the
stored track and album loudness:

- Web: GET /api/tracks/replay-gain?ids=... (up to 200), a lookup the
  player calls for its queue, rather than a field on every TrackRef
  surface.
- Android: track_gain/track_peak and album_gain/album_peak on the sync
  views, so cached tracks level offline. Storing a measurement logs a
  track change, and an album's values moving logs an album change, both
  before the write (#2704), so caches pick the gains up.
- OpenSubsonic: replayGain on every song (album, getSong, search3,
  starred), as a JSON object and an XML element.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/subsonic/star.go

@@ -191,6 +191,7 @@ func loadStarred(ctx context.Context, q *dbq.Queries, userID pgtype.UUID) ([]Son
 	if err != nil {
 		return nil, nil, nil, err
 	}
+	gains := replayGains(ctx, q, trackRows)
 	for _, t := range trackRows {
 		album, err := q.GetAlbumByID(ctx, t.AlbumID)
 		if err != nil {
@@ -200,7 +201,7 @@ func loadStarred(ctx context.Context, q *dbq.Queries, userID pgtype.UUID) ([]Son
 		if err != nil {
 			return nil, nil, nil, err
 		}
-		songs = append(songs, songRef(t, album.Title, artist.Name))
+		songs = append(songs, songRef(t, album.Title, artist.Name, gains[t.ID]))
 	}
 
 	albumRows, err := q.ListLikedAlbumRows(ctx, dbq.ListLikedAlbumRowsParams{