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/playback_reporting/sync_processor.rs

@@ -6,8 +6,8 @@
 #![allow(dead_code)]
 #![allow(unused_imports)]
 
-use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
 use std::time::Duration;
 use tokio::sync::Mutex as TokioMutex;
 
@@ -17,9 +17,9 @@ use crate::storage::db_service::RusqliteService;
 
 /// Configuration for sync processor
 pub struct SyncConfig {
-    pub max_retries: u32,          // 5
-    pub base_retry_delay_ms: u64,  // 1000ms
-    pub batch_size: usize,         // 10 items
+    pub max_retries: u32,         // 5
+    pub base_retry_delay_ms: u64, // 1000ms
+    pub batch_size: usize,        // 10 items
 }
 
 impl Default for SyncConfig {