feat: wire up subtitle prefetch service to MPV events

Initializes prefetch on external subtitle track activation, detects
seeks via time-pos delta threshold, pauses prefetch during live
subtitle processing, and restarts on cache invalidation.

- Extract loadSubtitleSourceText into reusable function
- Add prefetch service state and initSubtitlePrefetch helper
- Thread onTimePosUpdate through event actions/bindings/main-deps
- Pause prefetch on subtitle change, resume on emit
- Restart prefetch after tokenization cache invalidation
- Query track-list on media path change to find external subs

src/main/runtime/mpv-main-event-actions.ts

@@ -90,11 +90,13 @@ export function createHandleMpvTimePosChangeHandler(deps: {
   recordPlaybackPosition: (time: number) => void;
   reportJellyfinRemoteProgress: (forceImmediate: boolean) => void;
   refreshDiscordPresence: () => void;
+  onTimePosUpdate?: (time: number) => void;
 }) {
   return ({ time }: { time: number }): void => {
     deps.recordPlaybackPosition(time);
     deps.reportJellyfinRemoteProgress(false);
     deps.refreshDiscordPresence();
+    deps.onTimePosUpdate?.(time);
   };
 }