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.test.ts

@@ -390,6 +390,38 @@ describe("RepositoryClient", () => {
       });
     });
 
+    it("should get audio-only stream URL for a video item (camelCase params)", async () => {
+      // TRACES: UR-040 | JA-032 | UT-061
+      const mockUrl = "https://server.com/Audio/item123/universal?AudioStreamIndex=2";
+      (invoke as any).mockResolvedValueOnce(mockUrl);
+
+      const url = await client.getAudioOnlyStreamUrlForVideo("item123", "source456", 193, 2);
+
+      expect(url).toBe(mockUrl);
+      expect(invoke).toHaveBeenCalledWith("repository_get_audio_only_stream_url_for_video", {
+        handle: "test-handle-123",
+        itemId: "item123",
+        mediaSourceId: "source456",
+        startTimeSeconds: 193,
+        audioStreamIndex: 2,
+      });
+    });
+
+    it("should default optional params to null for audio-only stream URL", async () => {
+      // TRACES: UR-040 | JA-032 | UT-061
+      (invoke as any).mockResolvedValueOnce("https://server.com/Audio/item123/universal");
+
+      await client.getAudioOnlyStreamUrlForVideo("item123");
+
+      expect(invoke).toHaveBeenCalledWith("repository_get_audio_only_stream_url_for_video", {
+        handle: "test-handle-123",
+        itemId: "item123",
+        mediaSourceId: null,
+        startTimeSeconds: null,
+        audioStreamIndex: null,
+      });
+    });
+
     it("should report playback progress", async () => {
       (invoke as any).mockResolvedValueOnce(undefined);