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/session_poller/mod.rs

@@ -7,14 +7,14 @@
 
 use crate::utils::lock::{MutexSafe, RwLockSafe};
 use log::{debug, info, warn};
-use std::sync::{Arc, Mutex, RwLock};
 use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+use std::sync::{Arc, Mutex, RwLock};
 use std::thread;
 use std::time::Duration;
 
 use crate::jellyfin::JellyfinClient;
-use crate::player::PlayerEventEmitter;
 use crate::playback_mode::{PlaybackMode, PlaybackModeManager};
+use crate::player::PlayerEventEmitter;
 
 /// Hint for adjusting poll frequency based on UI state
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -103,10 +103,8 @@ impl SessionPollerManager {
 
             while is_running.load(Ordering::Relaxed) {
                 // Calculate poll interval based on mode and hint
-                let new_interval = Self::calculate_interval(
-                    &mode_manager.get_mode(),
-                    *hint.read_safe(),
-                );
+                let new_interval =
+                    Self::calculate_interval(&mode_manager.get_mode(), *hint.read_safe());
 
                 interval_ms.store(new_interval, Ordering::Relaxed);
 
@@ -158,9 +156,7 @@ impl SessionPollerManager {
                         }
 
                         if let Some(em) = emitter.lock_safe().as_ref() {
-                            em.emit(crate::player::PlayerStatusEvent::SessionsUpdated {
-                                sessions,
-                            });
+                            em.emit(crate::player::PlayerStatusEvent::SessionsUpdated { sessions });
                         }
                     }
                     Err(e) => {
@@ -201,10 +197,7 @@ impl SessionPollerManager {
     /// album, duration and position to the media notification. Silently does
     /// nothing if the session isn't found or has no now-playing item (e.g. the
     /// remote stopped) - the next state change will refresh it.
-    fn push_remote_lockscreen(
-        sessions: &[crate::jellyfin::client::SessionInfo],
-        session_id: &str,
-    ) {
+    fn push_remote_lockscreen(sessions: &[crate::jellyfin::client::SessionInfo], session_id: &str) {
         // 100ns Jellyfin ticks -> milliseconds.
         const TICKS_PER_MS: i64 = 10_000;
 
@@ -251,7 +244,10 @@ impl SessionPollerManager {
         };
 
         if let Err(e) = crate::player::update_lockscreen_metadata(&meta) {
-            warn!("[SessionPoller] Failed to update lockscreen metadata: {}", e);
+            warn!(
+                "[SessionPoller] Failed to update lockscreen metadata: {}",
+                e
+            );
         }
     }
 
@@ -262,7 +258,7 @@ impl SessionPollerManager {
             PollingHint::CastDiscovery => 15000, // Slow discovery
             PollingHint::Normal => {
                 match mode {
-                    PlaybackMode::Remote { .. } => 2000,  // Fast in remote mode
+                    PlaybackMode::Remote { .. } => 2000, // Fast in remote mode
                     PlaybackMode::Local | PlaybackMode::Idle => 10000, // Default
                 }
             }
@@ -271,7 +267,10 @@ impl SessionPollerManager {
 
     /// Manually trigger a poll (for frontend refresh button)
     pub async fn poll_now(&self) -> Result<Vec<crate::jellyfin::client::SessionInfo>, String> {
-        let client = self.jellyfin_client.lock_safe().clone()
+        let client = self
+            .jellyfin_client
+            .lock_safe()
+            .clone()
             .ok_or("Jellyfin client not configured")?;
 
         client.get_sessions().await
@@ -302,7 +301,9 @@ mod tests {
         );
         assert_eq!(
             SessionPollerManager::calculate_interval(
-                &PlaybackMode::Remote { session_id: "test".to_string() },
+                &PlaybackMode::Remote {
+                    session_id: "test".to_string()
+                },
                 PollingHint::CastActive
             ),
             500
@@ -310,16 +311,24 @@ mod tests {
 
         // CastDiscovery hint should always be 15s regardless of mode
         assert_eq!(
-            SessionPollerManager::calculate_interval(&PlaybackMode::Idle, PollingHint::CastDiscovery),
-            15000
-        );
-        assert_eq!(
-            SessionPollerManager::calculate_interval(&PlaybackMode::Local, PollingHint::CastDiscovery),
+            SessionPollerManager::calculate_interval(
+                &PlaybackMode::Idle,
+                PollingHint::CastDiscovery
+            ),
             15000
         );
         assert_eq!(
             SessionPollerManager::calculate_interval(
-                &PlaybackMode::Remote { session_id: "test".to_string() },
+                &PlaybackMode::Local,
+                PollingHint::CastDiscovery
+            ),
+            15000
+        );
+        assert_eq!(
+            SessionPollerManager::calculate_interval(
+                &PlaybackMode::Remote {
+                    session_id: "test".to_string()
+                },
                 PollingHint::CastDiscovery
             ),
             15000
@@ -338,7 +347,9 @@ mod tests {
         // Remote mode -> 2s
         assert_eq!(
             SessionPollerManager::calculate_interval(
-                &PlaybackMode::Remote { session_id: "test".to_string() },
+                &PlaybackMode::Remote {
+                    session_id: "test".to_string()
+                },
                 PollingHint::Normal
             ),
             2000