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>

web/src/lib/api/normalization.ts

@@ -0,0 +1,42 @@
+import { api } from './client';
+
+// Loudness normalization preference (M464 #4998). One per user, stored on
+// the server so the web player, the Android app and casts agree.
+export type NormalizationMode = 'off' | 'auto' | 'track' | 'album';
+export type NormalizationTarget = -18 | -16 | -14;
+export type NormalizationBoost = 'headroom' | 'limiter';
+
+export type NormalizationPrefs = {
+  mode: NormalizationMode;
+  target_lufs: NormalizationTarget;
+  boost: NormalizationBoost;
+};
+
+// Must match library.DefaultNormalizationPrefs on the server.
+export const DEFAULT_NORMALIZATION: NormalizationPrefs = {
+  mode: 'auto',
+  target_lufs: -18,
+  boost: 'headroom'
+};
+
+export const NORMALIZATION_MODES: NormalizationMode[] = ['off', 'auto', 'track', 'album'];
+export const NORMALIZATION_TARGETS: NormalizationTarget[] = [-18, -16, -14];
+
+export function isNormalizationPrefs(v: unknown): v is NormalizationPrefs {
+  if (typeof v !== 'object' || v === null) return false;
+  const p = v as Record<string, unknown>;
+  return (
+    NORMALIZATION_MODES.includes(p.mode as NormalizationMode) &&
+    NORMALIZATION_TARGETS.includes(p.target_lufs as NormalizationTarget) &&
+    (p.boost === 'headroom' || p.boost === 'limiter')
+  );
+}
+
+export function getNormalization(): Promise<NormalizationPrefs> {
+  return api.get<NormalizationPrefs>('/api/me/normalization');
+}
+
+// The whole preference is sent every time; the server keeps the last write.
+export function putNormalization(p: NormalizationPrefs): Promise<NormalizationPrefs> {
+  return api.put<NormalizationPrefs>('/api/me/normalization', p);
+}