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/commands/player/mod.rs

@@ -205,6 +205,15 @@ pub struct PlayItemRequest {
     /// zero-duration session renders no scrubber, even with ACTION_SEEK_TO set.
     #[serde(default)]
     pub duration_seconds: Option<f64>,
+    /// Item type (e.g. "Episode", "Movie", "Audio"). Carried through the
+    /// background-audio handoff so an episode played as audio-only is still
+    /// recognised as an episode by autoplay (UR-040) and advances to the next one.
+    #[serde(default)]
+    pub item_type: Option<String>,
+    /// Series ID for TV episodes. Needed alongside `item_type` so the backend can
+    /// look up the next episode when a background-audio track ends.
+    #[serde(default)]
+    pub series_id: Option<String>,
 }
 
 /// Queue context for remote transfer - what type of queue is this?
@@ -601,7 +610,9 @@ pub async fn player_enter_background_audio(
         artists: None,
         primary_image_tag: item.primary_image_tag.clone(),
         image_id: item.primary_image_tag.clone(),
-        item_type: None,
+        // Carry episode identity so autoplay can advance to the next episode when
+        // this audio-only handoff ends while backgrounded (UR-040).
+        item_type: item.item_type.clone(),
         playlist_id: None,
         // Carry the real duration so the lockscreen MediaSession can draw a scrubber.
         duration: item.duration_seconds,
@@ -616,7 +627,7 @@ pub async fn player_enter_background_audio(
         video_width: None,
         video_height: None,
         subtitles: vec![],
-        series_id: None,
+        series_id: item.series_id.clone(),
         server_id: item.server_id.clone(),
     };