fix(overlay): drive timing review playhead from mpv playback position

- Preview session observes time-pos, waits for loaded media before seeking, and acks each command
- Forward positions to the modal over media-timing-review:preview-position
- Cursor follows the real position, pauses while buffering, and stays at the clip end when done
- Replace the fixed end-of-clip timer with a 15s stall timeout
- Document preview cursor behavior and ownership in docs

src/types/anki.ts

@@ -82,6 +82,11 @@ export interface MediaTimingReviewPreviewRequest {
   endTime: number;
 }
 
+export interface MediaTimingReviewPreviewPosition {
+  reviewId: string;
+  time: number;
+}
+
 export interface MediaTimingReviewWaveformRequest {
   reviewId: string;
   startTime: number;