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/player/session.rs

@@ -1,3 +1,4 @@
+use super::media::MediaItem;
 /**
  * Media Session Management
  *
@@ -7,10 +8,8 @@
  *
  * See docs/architecture/01-rust-backend.md for the state machine diagram.
  */
-
 use log::info;
 use serde::{Deserialize, Serialize};
-use super::media::MediaItem;
 
 /// Media session type tracking the high-level playback context
 #[derive(specta::Type, Debug, Clone, Serialize, Deserialize, PartialEq)]
@@ -98,7 +97,10 @@ impl MediaSessionManager {
     /// Start an audio session with a queue
     /// Transitions: Idle → Audio(active), Any → Audio(active)
     pub fn start_audio_session(&mut self, first_item: MediaItem) {
-        info!("[MediaSession] Starting audio session: {}", first_item.title);
+        info!(
+            "[MediaSession] Starting audio session: {}",
+            first_item.title
+        );
         self.current = MediaSessionType::Audio {
             last_item: Some(first_item),
             is_active: true,
@@ -107,7 +109,11 @@ impl MediaSessionManager {
 
     /// Update audio session with new track (during playback)
     pub fn update_audio_track(&mut self, item: MediaItem) {
-        if let MediaSessionType::Audio { last_item, is_active } = &mut self.current {
+        if let MediaSessionType::Audio {
+            last_item,
+            is_active,
+        } = &mut self.current
+        {
             info!("[MediaSession] Updating audio track: {}", item.title);
             *last_item = Some(item);
             *is_active = true;
@@ -171,8 +177,14 @@ impl MediaSessionManager {
 
     /// Advance to next episode in TV session
     pub fn tv_session_next_episode(&mut self, next_item: MediaItem) {
-        if let MediaSessionType::TvShow { item, is_active, .. } = &mut self.current {
-            info!("[MediaSession] Advancing to next episode: {}", next_item.title);
+        if let MediaSessionType::TvShow {
+            item, is_active, ..
+        } = &mut self.current
+        {
+            info!(
+                "[MediaSession] Advancing to next episode: {}",
+                next_item.title
+            );
             *item = next_item;
             *is_active = true;
         }
@@ -294,7 +306,10 @@ mod tests {
 
         assert!(matches!(
             manager.current(),
-            MediaSessionType::Audio { is_active: true, .. }
+            MediaSessionType::Audio {
+                is_active: true,
+                ..
+            }
         ));
         assert!(manager.should_show_miniplayer());
 
@@ -307,7 +322,10 @@ mod tests {
         manager.audio_session_inactive();
         assert!(matches!(
             manager.current(),
-            MediaSessionType::Audio { is_active: false, .. }
+            MediaSessionType::Audio {
+                is_active: false,
+                ..
+            }
         ));
         assert!(manager.should_show_miniplayer()); // Still shows!
 
@@ -330,7 +348,10 @@ mod tests {
 
         assert!(matches!(
             manager.current(),
-            MediaSessionType::Movie { is_active: true, .. }
+            MediaSessionType::Movie {
+                is_active: true,
+                ..
+            }
         ));
         assert!(manager.should_show_video_player());