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_views.go

@@ -18,6 +18,7 @@ package api
 
 import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
@@ -49,9 +50,14 @@ type albumSyncView struct {
 	ReleaseDate  *string `json:"release_date"`
 	CoverArtPath *string `json:"cover_art_path"`
 	Mbid         *string `json:"mbid"`
+	// AlbumGain and AlbumPeak are the album's ReplayGain 2.0 values (#4997):
+	// dB to the -18 LUFS reference, and a linear peak. Null until every track
+	// on the album is measured.
+	AlbumGain *float32 `json:"album_gain"`
+	AlbumPeak *float32 `json:"album_peak"`
 }
 
-func toAlbumSyncView(a dbq.Album) albumSyncView {
+func toAlbumSyncView(a dbq.Album, g library.ReplayGain) albumSyncView {
 	var releaseDate *string
 	if a.ReleaseDate.Valid {
 		s := a.ReleaseDate.Time.Format("2006-01-02")
@@ -65,6 +71,8 @@ func toAlbumSyncView(a dbq.Album) albumSyncView {
 		ReleaseDate:  releaseDate,
 		CoverArtPath: a.CoverArtPath,
 		Mbid:         a.Mbid,
+		AlbumGain:    g.AlbumGain,
+		AlbumPeak:    g.AlbumPeak,
 	}
 }
 
@@ -92,9 +100,14 @@ type trackSyncView struct {
 	// track is playable, which is a yes/no. The "gone since" clock is an
 	// operator concern and lives on the admin surface.
 	Missing bool `json:"missing"`
+	// TrackGain and TrackPeak are the track's ReplayGain 2.0 values (#4997).
+	// Null until the track is measured. The album's pair rides on the album
+	// view, since it changes when the album does, not when this track does.
+	TrackGain *float32 `json:"track_gain"`
+	TrackPeak *float32 `json:"track_peak"`
 }
 
-func toTrackSyncView(t dbq.Track) trackSyncView {
+func toTrackSyncView(t dbq.Track, g library.ReplayGain) trackSyncView {
 	return trackSyncView{
 		ID:          syncpkg.FormatUUID(t.ID),
 		AlbumID:     syncpkg.FormatUUID(t.AlbumID),
@@ -107,6 +120,8 @@ func toTrackSyncView(t dbq.Track) trackSyncView {
 		FileFormat:  t.FileFormat,
 		Genre:       t.Genre,
 		Missing:     t.MissingSince.Valid,
+		TrackGain:   g.TrackGain,
+		TrackPeak:   g.TrackPeak,
 	}
 }