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/state.rs

@@ -199,7 +199,9 @@ mod tests {
     #[test]
     fn test_player_state_loading() {
         let media = create_test_media_item("item-1", "Test Item");
-        let state = PlayerState::Loading { media: media.clone() };
+        let state = PlayerState::Loading {
+            media: media.clone(),
+        };
         assert!(matches!(state, PlayerState::Loading { .. }));
         assert_eq!(state.position(), None);
         assert!(!state.is_playing());