Background-audio handoff for video + repository/player refactor

Hand video playback off to a native audio-only stream when the app is
backgrounded or locked, with no on-device video decode (UR-040). Adds
player_enter/exit_background_audio commands, an audio-only stream URL
for video items across the repository layer, and the frontend handoff
state machine wired into VideoPlayer. Includes accompanying
repository/offline/player refactoring and regenerates the traceability
matrix.

src/lib/api/repository-client.ts

@@ -172,6 +172,26 @@ export class RepositoryClient {
     );
   }
 
+  /**
+   * Audio-only stream URL for a video item, for the background-audio handoff.
+   * The server extracts just the audio track — no video is decoded on-device.
+   * TRACES: UR-040 | JA-032
+   */
+  async getAudioOnlyStreamUrlForVideo(
+    itemId: string,
+    mediaSourceId?: string,
+    startTimeSeconds?: number,
+    audioStreamIndex?: number
+  ): Promise<string> {
+    return commands.repositoryGetAudioOnlyStreamUrlForVideo(
+      this.ensureHandle(),
+      itemId,
+      mediaSourceId ?? null,
+      startTimeSeconds ?? null,
+      audioStreamIndex ?? null
+    );
+  }
+
   // ===== Live TV / Channels =====
 
   /** Browse Live TV channels (broadcast / IPTV). */