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/library/loudness_backfill.go

@@ -187,7 +187,7 @@ func (w *LoudnessBackfillWorker) pass(ctx context.Context) (BackfillLoudnessResu
 						w.logger.Error("loudness backfill: track panicked", "path", row.FilePath, "panic", r)
 					}
 				}()
-				outcome := storeLoudness(ctx, q, w.logger, row.ID, row.FilePath,
+				outcome := storeLoudness(ctx, w.pool, w.logger, row.ID, row.FilePath,
 					w.analyzeFile(ctx, row.FilePath, row.DurationMs))
 				mu.Lock()
 				res.add(outcome)