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/download/pinning.rs

@@ -1,4 +1,6 @@
 //! Pinning commands - protect an item's cached metadata from cache clearing.
+//!
+//! TRACES: UR-044 | DR-056
 
 use std::sync::Arc;
 use tauri::State;
@@ -45,7 +47,10 @@ pub async fn unpin_item(db: State<'_, DatabaseWrapper>, item_id: String) -> Resu
 /// Check if an item is pinned
 #[tauri::command]
 #[specta::specta]
-pub async fn is_item_pinned(db: State<'_, DatabaseWrapper>, item_id: String) -> Result<bool, String> {
+pub async fn is_item_pinned(
+    db: State<'_, DatabaseWrapper>,
+    item_id: String,
+) -> Result<bool, String> {
     let db_service = {
         let database = db.0.lock().map_err(|e| e.to_string())?;
         Arc::new(database.service())