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/download/cache.rs

@@ -36,7 +36,7 @@ impl Default for CacheConfig {
             album_affinity_enabled: true,
             album_affinity_threshold: 3,
             storage_limit: 10 * 1024 * 1024 * 1024, // 10GB
-            wifi_only: false, // Allow preloading on any connection by default
+            wifi_only: false,                       // Allow preloading on any connection by default
         }
     }
 }
@@ -225,7 +225,10 @@ impl SmartCache {
                 "DELETE FROM downloads WHERE id = ?",
                 vec![QueryParam::Int64(id)],
             );
-            db_service.execute(delete_query).await.map_err(|e| e.to_string())?;
+            db_service
+                .execute(delete_query)
+                .await
+                .map_err(|e| e.to_string())?;
 
             freed += size as u64;
         }