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/components/player/VideoPlayer.svelte

@@ -1233,6 +1233,10 @@
           serverId: media.serverId ?? null,
           // Real duration so the lockscreen scrubber has a range to draw.
           durationSeconds: duration > 0 ? duration : null,
+          // Episode identity so the backend can auto-advance to the next episode
+          // when this audio-only stream ends while backgrounded (UR-040).
+          itemType: media.type ?? null,
+          seriesId: media.seriesId ?? null,
         },
         pos,
       );