fix(downloads): a conversion's commit, a stopped upload, the fast join and a clip's copy hold up

- The fast join gives each ffconcat entry its own media-only format whitelist, so a library file
  can no longer be opened as a nested concat list (the list's whitelist was handed down to it).
- A file the guarded probe cannot read is a download-only file, not a guess from yt-dlp's info.
- A conversion's or an ingest's commit that raises is re-read in a fresh session before any file
  is removed: a commit that landed keeps the file its row now names.
- An original that cannot be removed after conversion is logged; the upload stays ready.
- An upload whose conversion is cancelled or deleted midway is freed once nothing holds it; a
  paused one is kept.
- A shared link says "preparing" only while its video's job is queued or running.
- The admin storage view counts every file on disk ("Files on disk"), not only ready ones.
- A clip's failure title, unknown failure and cancel dialog, and a clip's and an upload's delete
  dialog speak of what the job made; an upload's quota refusal explains that the original is kept.
- /watch announces "preparing" as an alert and moves focus to the new Retry after an action.

backend/app/routes/public.py

@@ -16,6 +16,7 @@ from app.config import settings
 from app.db import get_db
 from app.downloads import links as linksmod
 from app.downloads import storage
+from app.downloads.status import JobStatus
 from app.models import DownloadJob, DownloadLink, MediaAsset
 from app.ratelimit import RateLimiter, client_key
 from app.security import verify_password
@@ -46,9 +47,16 @@ 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"):
+    if (
+        job is not None
+        and asset is not None
+        and asset.status in ("pending", "downloading")
+        and job.status in (JobStatus.QUEUED, JobStatus.RUNNING)
+    ):
         # 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".
+        # Only while the job is still going: a cancelled or paused rebuild leaves the asset pending
+        # with nothing working on it, and "try again in a few minutes" would never come true.
         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")