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/api/library_sync.go

@@ -10,6 +10,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
@@ -151,8 +152,12 @@ func (h *handlers) hydrateUpserts(
 		if err != nil {
 			return nil, err
 		}
+		gains, err := library.ReplayGainForAlbums(ctx, q, uuids)
+		if err != nil {
+			return nil, err
+		}
 		for _, a := range albums {
-			b, _ := json.Marshal(toAlbumSyncView(a))
+			b, _ := json.Marshal(toAlbumSyncView(a, gains[a.ID]))
 			out["album"] = append(out["album"], b)
 		}
 	}
@@ -162,8 +167,12 @@ func (h *handlers) hydrateUpserts(
 		if err != nil {
 			return nil, err
 		}
+		gains, err := library.ReplayGainForTracks(ctx, q, uuids)
+		if err != nil {
+			return nil, err
+		}
 		for _, t := range tracks {
-			b, _ := json.Marshal(toTrackSyncView(t))
+			b, _ := json.Marshal(toTrackSyncView(t, gains[t.ID]))
 			out["track"] = append(out["track"], b)
 		}
 	}