feat: integrated audio player + animated transcript follower

- Video detail (done videos): added 'Download audio' button; removed the
  manual 'Thumbnail' button (thumbnails auto-download now, so it was redundant).
- New 'GET /api/videos/{id}/audio' endpoint (also serves HEAD for probing),
  serving data/audio/<video_id>.mp3. Audio job outtmpl switched to %(id)s so
  files are addressable per video.
- Integrated <audio> player appears in the detail view once the MP3 is present
  (probed via HEAD on openVideo; polled after a download job until ready).
- Synced transcript follower: as audio plays, the matching segment is
  highlighted (seg-active) and auto-scrolled into view like a karaoke/lyrics
  tracker. Clicking any segment timestamp or chapter seeks the audio to that
  point (falls back to scroll when no audio). Playback indicator pulses while
  playing.

src/yt_scraper/store.py

@@ -575,6 +575,22 @@ class Store:
             cur.execute("SELECT * FROM scrape_jobs ORDER BY started_at DESC LIMIT ?", (limit,))
             return [_row_to_jobrow(r) for r in cur.fetchall()]
 
+    TERMINAL_STATUSES = ("done", "error", "cancelled")
+
+    def delete_job(self, job_id: str) -> bool:
+        with self._cursor() as cur:
+            cur.execute("DELETE FROM scrape_jobs WHERE id = ?", (job_id,))
+            return cur.rowcount > 0
+
+    def delete_terminal_jobs(self) -> int:
+        placeholders = ",".join("?" for _ in self.TERMINAL_STATUSES)
+        with self._cursor() as cur:
+            cur.execute(
+                f"DELETE FROM scrape_jobs WHERE status IN ({placeholders})",
+                list(self.TERMINAL_STATUSES),
+            )
+            return cur.rowcount
+
     # ------------------------------------------------------------------ aggregates
 
     def stats(self, channel_id: str | None = None) -> dict[str, int]: