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/commands/device.rs

@@ -81,7 +81,10 @@ pub async fn device_get_id(db: State<'_, DatabaseWrapper>) -> Result<String, Str
 /// TRACES: UR-009 | DR-011
 #[tauri::command]
 #[specta::specta]
-pub async fn device_set_id(device_id: String, db: State<'_, DatabaseWrapper>) -> Result<(), String> {
+pub async fn device_set_id(
+    device_id: String,
+    db: State<'_, DatabaseWrapper>,
+) -> Result<(), String> {
     let db_service = {
         let database = db.0.lock().map_err(|e| e.to_string())?;
         Arc::new(database.service())