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/db/queries/loudness.sql

@@ -133,11 +133,39 @@ ON CONFLICT (album_id) DO UPDATE SET
     inputs_digest   = EXCLUDED.inputs_digest,
     computed_at     = now();
 
--- name: DeleteOrphanAlbumLoudness :execrows
+-- name: ListOrphanAlbumLoudness :many
 -- Album rows whose album has no present track left (every file missing). The
 -- album row itself survives a missing file, so its loudness would otherwise
 -- stay behind describing tracks that are gone; it is recomputed if they return.
-DELETE FROM album_loudness a
+SELECT a.album_id
+  FROM album_loudness a
  WHERE NOT EXISTS (
      SELECT 1 FROM tracks t WHERE t.album_id = a.album_id AND t.missing_since IS NULL
  );
+
+-- name: DeleteAlbumLoudness :exec
+DELETE FROM album_loudness WHERE album_id = ANY(sqlc.arg(album_ids)::uuid[]);
+
+-- name: GetAlbumLoudness :one
+-- The stored album values, read before a recompute so a sync change is
+-- logged only when they actually move.
+SELECT integrated_lufs, true_peak_dbtp FROM album_loudness WHERE album_id = $1;
+
+-- name: GetReplayGainByTrackIDs :many
+-- Track and album loudness for a set of tracks, for every surface that hands
+-- gains to a client (#4997). A measurement from an older analysis version is
+-- still delivered: it is a better gain than none until the backfill redoes it.
+SELECT t.id,
+       tl.integrated_lufs AS track_lufs,
+       tl.true_peak_dbtp  AS track_peak_dbtp,
+       al.integrated_lufs AS album_lufs,
+       al.true_peak_dbtp  AS album_peak_dbtp
+  FROM tracks t
+  LEFT JOIN track_loudness tl ON tl.track_id = t.id
+  LEFT JOIN album_loudness al ON al.album_id = t.album_id
+ WHERE t.id = ANY(sqlc.arg(ids)::uuid[]);
+
+-- name: GetAlbumLoudnessByIDs :many
+SELECT album_id, integrated_lufs, true_peak_dbtp
+  FROM album_loudness
+ WHERE album_id = ANY(sqlc.arg(ids)::uuid[]);