feat(web): level playback by the user's normalization preference (M464 #4999)

Cuts go through element.volume. Boosts route the element through a Web
Audio GainNode and a DynamicsCompressor (a -1 dBFS limiter in limiter
mode, a pass-through otherwise), built only when a track wants a boost
and only once an AudioContext is confirmed running; iOS never gets the
graph. Auto mode takes album gain when a queue neighbour is from the
same album in track order. Gains are fetched for the next 50 tracks as
the queue moves, with a 10s deadline. The prefetch element is untouched.

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

web/src/lib/api/normalization.ts

@@ -1,4 +1,4 @@
-import { api } from './client';
+import { api, apiFetch } from './client';
 
 // Loudness normalization preference (M464 #4998). One per user, stored on
 // the server so the web player, the Android app and casts agree.
@@ -40,3 +40,23 @@ export function getNormalization(): Promise<NormalizationPrefs> {
 export function putNormalization(p: NormalizationPrefs): Promise<NormalizationPrefs> {
   return api.put<NormalizationPrefs>('/api/me/normalization', p);
 }
+
+// Per-track gains (#4997): dB to the -18 LUFS ReplayGain reference and linear
+// peaks. A field is absent until the server has measured it.
+export type ReplayGain = {
+  track_gain?: number;
+  track_peak?: number;
+  album_gain?: number;
+  album_peak?: number;
+};
+
+/**
+ * Gains for up to 200 tracks; an id missing from items has none yet. Given
+ * a deadline: the player waits on it, and a request that never answers
+ * would leave those tracks unleveled for the rest of the session.
+ */
+export function getReplayGain(ids: string[]): Promise<{ items: Record<string, ReplayGain> }> {
+  return apiFetch(`/api/tracks/replay-gain?ids=${ids.map(encodeURIComponent).join(',')}`, {
+    signal: AbortSignal.timeout(10_000)
+  }) as Promise<{ items: Record<string, ReplayGain> }>;
+}