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/browse.go

@@ -185,9 +185,10 @@ func (b *browseHandlers) getAlbum(w http.ResponseWriter, r *http.Request) {
 		WriteFail(w, r, ErrGeneric, "Failed to load tracks")
 		return
 	}
+	gains := replayGains(r.Context(), q, tracks)
 	songs := make([]SongRef, 0, len(tracks))
 	for _, t := range tracks {
-		songs = append(songs, songRef(t, album.Title, artist.Name))
+		songs = append(songs, songRef(t, album.Title, artist.Name, gains[t.ID]))
 	}
 	Write(w, r, AlbumResponse{
 		Envelope: NewEnvelope("ok"),
@@ -230,7 +231,7 @@ func (b *browseHandlers) getSong(w http.ResponseWriter, r *http.Request) {
 	}
 	Write(w, r, SongResponse{
 		Envelope: NewEnvelope("ok"),
-		Song:     songRef(track, album.Title, artist.Name),
+		Song:     songRef(track, album.Title, artist.Name, replayGains(r.Context(), q, []dbq.Track{track})[track.ID]),
 	})
 }
 
@@ -386,6 +387,7 @@ func (b *browseHandlers) search3(w http.ResponseWriter, r *http.Request) {
 			WriteFail(w, r, ErrGeneric, "Song search failed")
 			return
 		}
+		gains := replayGains(r.Context(), q, tracks)
 		for _, t := range tracks {
 			album, aerr := q.GetAlbumByID(r.Context(), t.AlbumID)
 			if aerr != nil {
@@ -397,7 +399,7 @@ func (b *browseHandlers) search3(w http.ResponseWriter, r *http.Request) {
 				WriteFail(w, r, ErrGeneric, "Song search failed")
 				return
 			}
-			result.Songs = append(result.Songs, songRef(t, album.Title, artist.Name))
+			result.Songs = append(result.Songs, songRef(t, album.Title, artist.Name, gains[t.ID]))
 		}
 	}