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/lib/api/bindings.ts

@@ -2055,7 +2055,18 @@ artist?: string | null; primaryImageTag?: string | null; serverId?: string | nul
  * handoff so the lockscreen MediaSession advertises a real duration — a
  * zero-duration session renders no scrubber, even with ACTION_SEEK_TO set.
  */
-durationSeconds?: number | null }
+durationSeconds?: number | null; 
+/**
+ * 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.
+ */
+itemType?: string | null; 
+/**
+ * Series ID for TV episodes. Needed alongside `item_type` so the backend can
+ * look up the next episode when a background-audio track ends.
+ */
+seriesId?: string | null }
 /**
  * Queue context for remote transfer - what type of queue is this?
  */