Fix download status handling and the blank page after download

- Never let a non-authoritative MeTube 'updated' event downgrade a
  terminal job (it refilled the progress bar after completion).
- Self-heal stale active jobs against MeTube history on status polls,
  so a missed event no longer leaves a job stuck in 'queued'.
- Build YT.Player on an imperatively created child div: React keeps
  owning the container, so switching to the local copy after a
  download no longer throws removeChild and blanks the page.
- Autoplay on open and seek controls; subtitle experiment reverted.

backend/app/api/videos.py

@@ -14,6 +14,8 @@ from app.services.download_jobs import (
     MeTubeRejected,
     delete_local_copy,
     get_latest_job,
+    job_is_stale,
+    reconcile_stale_job,
     request_download,
 )
 from app.services.metube_client import MeTubeClient
@@ -100,6 +102,15 @@ def download_status(youtube_video_id: str, db: Session = Depends(get_db)) -> dic
     job = get_latest_job(db, video.id)
     if job is None:
         return {"status": "not_downloaded", "progress_percent": None, "media_url": None, "error_message": None}
+    # Lazy self-heal: an active job whose Socket.IO events stopped (e.g. the
+    # terminal 'completed' event was missed while we were down) is reconciled
+    # against MeTube history on poll -- at most once per stale window.
+    if job_is_stale(job):
+        try:
+            reconcile_stale_job(db, job)
+        except Exception:
+            logger.warning("Could not self-heal stale download job %s", job.id, exc_info=True)
+            db.rollback()
     return _serialize_job(job)