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-tauri/src/player/mpv_backend_test.rs

@@ -86,7 +86,10 @@ mod tests {
         std::thread::sleep(std::time::Duration::from_millis(100));
 
         let count = *counter.lock().unwrap();
-        assert_eq!(count, 1, "Fallback pattern should execute async code successfully");
+        assert_eq!(
+            count, 1,
+            "Fallback pattern should execute async code successfully"
+        );
     }
 
     /// Test that position update logic works in a thread
@@ -113,7 +116,11 @@ mod tests {
         handle.join().unwrap();
 
         let recorded_positions = positions.lock().unwrap();
-        assert_eq!(recorded_positions.len(), 5, "Should have recorded 5 position updates");
+        assert_eq!(
+            recorded_positions.len(),
+            5,
+            "Should have recorded 5 position updates"
+        );
 
         // Verify positions are increasing
         for (i, pos) in recorded_positions.iter().enumerate() {