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/api.go

@@ -132,6 +132,9 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Get("/library/genres", h.handleListGenres)
 			authed.Get("/library/years", h.handleListAlbumYears)
 			authed.Get("/library/sync", h.handleLibrarySync)
+			// Before /tracks/{id} for readability; chi prefers the static
+			// segment either way.
+			authed.Get("/tracks/replay-gain", h.handleGetReplayGain)
 			authed.Get("/tracks/{id}", h.handleGetTrack)
 			// /tracks/{id}/stream is mounted above with OptionalUser so
 			// it can accept either a session or a signed token.