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/tests/test_download_errors.py

@@ -64,6 +64,7 @@ def test_our_own_worker_refusals_classify_as_themselves():
     # from silently orphaning the category — the UI would then fall back to "Download failed".
     assert classify(dlerrors.QUOTA_FULL_MESSAGE) == "quota_exceeded"
     assert classify(dlerrors.CLIP_TOO_BIG_MESSAGE) == "quota_exceeded"
+    assert classify(dlerrors.CONVERTED_TOO_BIG_MESSAGE) == "quota_exceeded"
 
 
 def test_the_redacted_network_text_classifies_back_to_network():