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/lib/api/bindings.ts

@@ -19,6 +19,40 @@ export const commands = {
 async playerPlayItem(item: PlayItemRequest) : Promise<PlayerStatus> {
     return await TAURI_INVOKE("player_play_item", { item });
 },
+/**
+ * Enter background-audio mode: hand playback of the currently-watched video off
+ * to the native ExoPlayer *audio* path so the audio keeps playing while the app
+ * is backgrounded/locked, with no client-side video decode (UR-040).
+ * 
+ * `stream_url` MUST be an audio-only URL (see
+ * `get_audio_only_stream_url_for_video`). The item is created as
+ * `MediaType::Audio` so it starts an audio session and loads into the native
+ * backend with `mediaType="audio"` — the WebView `<video>` is torn down on the
+ * frontend side, so exactly one audio source is ever active.
+ * 
+ * This deliberately goes through the queue-based `play_item` path (NOT a
+ * side-channel) so end-of-track lands in `on_playback_ended`, which already
+ * honors the sleep timer (Time/Episodes/EndOfTrack) and drives autoplay-next.
+ * The sleep-timer state is intentionally left untouched by the handoff.
+ * 
+ * TRACES: UR-040 | DR-052 | UT-061, IT-013
+ */
+async playerEnterBackgroundAudio(item: PlayItemRequest, positionSeconds: number) : Promise<PlayerStatus> {
+    return await TAURI_INVOKE("player_enter_background_audio", { item, positionSeconds });
+},
+/**
+ * Exit background-audio mode: stop the native audio player and return its final
+ * position so the frontend can reload the WebView `<video>` there (UR-040).
+ * 
+ * Returns the position in seconds. The sleep timer is intentionally left
+ * untouched — if it fired while backgrounded, playback is already stopped and
+ * this simply reports the last position.
+ * 
+ * TRACES: UR-040 | DR-052 | UT-061, IT-013
+ */
+async playerExitBackgroundAudio() : Promise<number> {
+    return await TAURI_INVOKE("player_exit_background_audio");
+},
 /**
  * Play a queue of media items
  * 
@@ -1206,6 +1240,14 @@ async repositoryGetVideoStreamUrl(handle: string, itemId: string, mediaSourceId:
 async repositoryGetAudioStreamUrl(handle: string, itemId: string) : Promise<string> {
     return await TAURI_INVOKE("repository_get_audio_stream_url", { handle, itemId });
 },
+/**
+ * Get an audio-only stream URL for a *video* item (background-audio handoff).
+ * 
+ * TRACES: UR-040 | JA-032 | UT-061
+ */
+async repositoryGetAudioOnlyStreamUrlForVideo(handle: string, itemId: string, mediaSourceId: string | null, startTimeSeconds: number | null, audioStreamIndex: number | null) : Promise<string> {
+    return await TAURI_INVOKE("repository_get_audio_only_stream_url_for_video", { handle, itemId, mediaSourceId, startTimeSeconds, audioStreamIndex });
+},
 /**
  * Get Live TV channels (broadcast / IPTV) for browsing
  */
@@ -1760,7 +1802,13 @@ videoCodec: string;
 /**
  * Whether the video requires server-side transcoding
  */
-needsTranscoding: boolean }
+needsTranscoding: boolean; 
+/**
+ * Optional now-playing metadata. Used by the background-audio handoff so the
+ * lockscreen/miniplayer show the item (title/subtitle/artwork). Defaulted so
+ * existing video-only callers need not send them.
+ */
+artist?: string | null; primaryImageTag?: string | null; serverId?: string | null }
 /**
  * Queue context for remote transfer - what type of queue is this?
  */