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/player/backgroundAudio.commands.test.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { invoke } from "@tauri-apps/api/core";
+import { commands } from "$lib/api/bindings";
+
+vi.mock("@tauri-apps/api/core");
+
+// TRACES: UR-040 | DR-052 | UT-061
+//
+// Guards the Tauri v2 camelCase param rule for the background-audio commands:
+// the command NAME stays snake_case; params are camelCase.
+
+describe("background-audio player commands (param naming)", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("player_enter_background_audio sends item + positionSeconds (camelCase)", async () => {
+    (invoke as any).mockResolvedValueOnce({});
+
+    const item = {
+      id: "vid-1",
+      title: "Episode 1",
+      streamUrl: "https://server/Audio/vid-1/universal?AudioStreamIndex=1",
+      videoCodec: "aac",
+      needsTranscoding: false,
+    };
+    await commands.playerEnterBackgroundAudio(item, 193);
+
+    expect(invoke).toHaveBeenCalledWith("player_enter_background_audio", {
+      item,
+      positionSeconds: 193,
+    });
+  });
+
+  it("player_exit_background_audio takes no params and returns a position", async () => {
+    (invoke as any).mockResolvedValueOnce(193.5);
+
+    const pos = await commands.playerExitBackgroundAudio();
+
+    expect(pos).toBe(193.5);
+    expect(invoke).toHaveBeenCalledWith("player_exit_background_audio");
+  });
+});