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/mod.rs

@@ -11,8 +11,8 @@ pub mod events;
 pub mod worker;
 
 use crate::utils::lock::MutexSafe;
-use std::path::PathBuf;
 use std::collections::HashSet;
+use std::path::PathBuf;
 use std::sync::{Arc, Mutex};
 
 pub use worker::DownloadWorker;