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.go

@@ -17,6 +17,7 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
 // Loudness analysis (M464 #4995).
@@ -351,10 +352,15 @@ const (
 // An inconclusive attempt leaves any existing row alone. The scan deletes a
 // row when its file changes, so a row still here describes these bytes, and a
 // value from an older method is a better gain than none until it is redone.
+//
+// A stored measurement is logged to the sync feed as a track upsert, so
+// clients that cache the library (Android) pick up the new gain (#4997). If
+// the log fails nothing is stored, and the backfill tries the track again.
 func storeLoudness(
-	ctx context.Context, q *dbq.Queries, logger *slog.Logger,
+	ctx context.Context, db dbq.DBTX, logger *slog.Logger,
 	trackID pgtype.UUID, path string, r loudnessResult,
 ) loudnessOutcome {
+	q := dbq.New(db)
 	if r.err != nil {
 		logger.Warn("loudness: analysis failed", "path", path, "err", r.err)
 		if r.inconclusive() {
@@ -388,6 +394,14 @@ func storeLoudness(
 			}
 		}
 	}
+	// Logged before the write, as reconcile does (#2704): a log that then
+	// finds the write failed costs clients one wasted re-read, but a write
+	// that lands with no log is never retried, since the track is settled, and
+	// clients would never learn its gain.
+	if err := syncpkg.LogChange(ctx, db, syncpkg.EntityTrack, syncpkg.FormatUUID(trackID), syncpkg.OpUpsert); err != nil {
+		logger.Warn("loudness: LogChange track upsert failed; not storing", "path", path, "err", err)
+		return loudnessStoreFailed
+	}
 	if err := q.UpsertTrackLoudness(ctx, params); err != nil {
 		logger.Warn("loudness: storing measurement failed", "path", path, "err", err)
 		return loudnessStoreFailed