feat: per-user volume leveling preference, synced across devices (M464 #4998)

Mode (off, auto, track, album), target (-18, -16, -14 LUFS) and boost
(within headroom, or fully with a limiter), stored per user on the server
so the web player, the Android app and casts apply the same one.

- Server: user_normalization_prefs (migration 0067), GET/PUT
  /api/me/normalization; a whole-body PUT, validated, last write wins.
- Web: Settings > Playback > Volume leveling. Saves at once, restores the
  old choice if the save fails, and caches the value for the player.
- Android: Settings card. The device keeps a copy for offline playback
  (Room v9 with an explicit migration, so the upgrade wipes nothing).
  Writes are offline-first: shown at once, PUT best effort, queued on
  failure (NORMALIZATION_SET, collapsed to the newest). A refresh never
  overwrites a change still queued.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/api.go

@@ -109,6 +109,8 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Put("/me/password", h.handleChangePassword)
 			authed.Put("/me/profile", h.handleUpdateMyProfile)
 			authed.Put("/me/timezone", h.handlePutTimezone)
+			authed.Get("/me/normalization", h.handleGetMyNormalization)
+			authed.Put("/me/normalization", h.handlePutMyNormalization)
 			authed.Post("/me/api-token", h.handleRegenerateMyAPIToken)
 			authed.Get("/me/subsonic-password", h.handleGetMySubsonicPassword)
 			authed.Post("/me/subsonic-password", h.handleGenerateMySubsonicPassword)