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_test.go

@@ -11,6 +11,7 @@ import (
 	"testing"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
 // TestLoudnessBackfill_Integration pins which tracks a pass measures, what it
@@ -130,6 +131,18 @@ func TestLoudnessBackfill_Integration(t *testing.T) {
 			gotLUFS, gotPeak, gotHist, gotStart, gotVersion)
 	}
 
+	// Each stored measurement told the sync feed, so caching clients re-read
+	// the track and pick up its gain (#4997).
+	var logged int
+	if err := pool.QueryRow(ctx, `SELECT count(*) FROM library_changes
+	                               WHERE entity_type = 'track' AND op = 'upsert' AND entity_id = $1`,
+		syncpkg.FormatUUID(stale.ID)).Scan(&logged); err != nil {
+		t.Fatalf("count sync changes: %v", err)
+	}
+	if logged != 1 {
+		t.Errorf("re-measuring stale.mp3 logged %d track changes, want 1", logged)
+	}
+
 	// 2. A pass after a complete one is a no-op.
 	res, err = w.pass(ctx)
 	if err != nil {