fix(watch): a link whose video is being made again says so, in the page's language

The public watch route answers video_preparing for an asset that is pending or
downloading, and the page shows "being prepared — try again" with a Retry
instead of "this link is gone". watch.html ships one lang; the page now sets
documentElement.lang to the language its strings were picked in.

backend/app/routes/public.py

@@ -46,6 +46,10 @@ def _link_or_404(db: Session, token: str) -> DownloadLink:
 def _ready_target(db: Session, link: DownloadLink) -> tuple[DownloadJob, MediaAsset]:
     job = db.get(DownloadJob, link.job_id)
     asset = db.get(MediaAsset, job.asset_id) if job and job.asset_id else None
+    if asset is not None and asset.status in ("pending", "downloading"):
+        # Being (re)made — a re-download, or a conversion — so the link works again once it is ready:
+        # its own reason, so the page can say "try again shortly" rather than "this link is gone".
+        raise refused(404, "video_preparing")
     if job is None or asset is None or asset.status != "ready" or not asset.rel_path:
         raise refused(404, "video_gone")
     # Verify the file is actually on disk here (not just flagged ready), so the watch page never