fix(autoplay): advance to the next episode in background audio mode

An episode handed off to the audio-only path for background playback is a
MediaType::Audio item, so autoplay's video-only checks stopped
recognising it as an episode: playback simply ended at the episode
boundary instead of continuing to the next one.

- Carry episode identity (item_type, series_id) through the
  background-audio handoff so the backend queue item still knows it's an
  episode; is_episode_item now trusts item_type over the media_type
  heuristic, and the sleep timer's episode counter follows.
- The frontend normally performs the advance by navigating to
  /player/<id>, which is unavailable while the WebView is suspended.
  advance_to_next_episode_audio_only drives it entirely in the backend:
  fetch the next episode, build its audio-only stream URL, and load it
  into the native audio player, preserving episode identity so the
  following boundary advances too.
- Android's autoplay dispatch routes background-audio episodes to that
  backend advance and keeps the countdown path for the foreground.
- get_audio_only_stream_url_for_video joins the MediaRepository trait
  (online delegates to the existing builder, offline errors) so the
  controller can reach it without a frontend round-trip.

TRACES: UR-040, UR-023 | DR-052 | JA-032

src-tauri/src/repository/mod.rs

@@ -117,6 +117,22 @@ pub trait MediaRepository: Send + Sync {
     /// @req: JA-007 - Get playback info and stream URL
     async fn get_audio_stream_url(&self, item_id: &str) -> Result<String, RepoError>;
 
+    /// Get an audio-only stream URL for a *video* item (background-audio handoff).
+    ///
+    /// Used when autoplay advances to the next episode while the app is playing a
+    /// video in audio-only mode in the background: the backend needs the next
+    /// episode's audio-only URL without any frontend round-trip. Online-only;
+    /// offline/cache repositories return an error.
+    ///
+    /// TRACES: UR-040 | JA-032
+    async fn get_audio_only_stream_url_for_video(
+        &self,
+        item_id: &str,
+        media_source_id: Option<&str>,
+        start_time_seconds: Option<f64>,
+        audio_stream_index: Option<i32>,
+    ) -> Result<String, RepoError>;
+
     /// Get Live TV channels (broadcast / IPTV) for browsing.
     async fn get_live_tv_channels(&self) -> Result<Vec<MediaItem>, RepoError>;