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/backend.rs

@@ -163,7 +163,12 @@ impl PlayerBackend for NullBackend {
     }
 
     fn play(&mut self) -> Result<(), PlayerError> {
-        if let PlayerState::Paused { media, position, duration } = &self.state {
+        if let PlayerState::Paused {
+            media,
+            position,
+            duration,
+        } = &self.state
+        {
             self.state = PlayerState::Playing {
                 media: media.clone(),
                 position: *position,
@@ -174,7 +179,12 @@ impl PlayerBackend for NullBackend {
     }
 
     fn pause(&mut self) -> Result<(), PlayerError> {
-        if let PlayerState::Playing { media, position, duration } = &self.state {
+        if let PlayerState::Playing {
+            media,
+            position,
+            duration,
+        } = &self.state
+        {
             self.state = PlayerState::Paused {
                 media: media.clone(),
                 position: *position,