fix(downloads): a converted upload too big says so, and a suspended owner's waiting rebuild is not "preparing"

- A converted upload over the space left (not zero) fails with CONVERTED_TOO_BIG_MESSAGE, not
  "storage is full"; it classifies as quota_exceeded by the phrase it shares with the clip's.
- The public watch page says "preparing" only for a job a worker will take: a suspended owner's
  queued job waits for as long as the suspension lasts, so the link says the video is gone, as
  for a paused rebuild.
- The worker's claim docstring names the restart case: a running job put back in the queue waits.

backend/app/downloads/pipeline.py

@@ -1160,7 +1160,7 @@ def _check_converted_fits(owner_id: int | None, old_size: int, new_size: int) ->
         return
     with SessionLocal() as db:
         if not quota.fits_after_conversion(db, owner_id, old_size, new_size):
-            raise StatedFailure(dlerrors.QUOTA_FULL_MESSAGE)
+            raise StatedFailure(dlerrors.CONVERTED_TOO_BIG_MESSAGE)
 
 
 def _unplace(placed: Path | None) -> None: