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/errors.py

@@ -37,9 +37,10 @@ from app.downloads.credentials import host_key_for_url
 
 # Worker-authored strings that this module owns because each is coupled to `_RULES`.
 #
-# QUOTA_FULL_MESSAGE and CLIP_TOO_BIG_MESSAGE are raised by the worker and must classify back to
-# `quota_exceeded`; their rules below match a fragment of each, and test_download_errors pins the pair, because a message and its
-# rule drifting apart is exactly how a category silently becomes unreachable.
+# QUOTA_FULL_MESSAGE, CLIP_TOO_BIG_MESSAGE and CONVERTED_TOO_BIG_MESSAGE are raised by the worker and
+# must classify back to `quota_exceeded`; their rules below match a fragment of each, and
+# test_download_errors pins each one, because a message and its rule drifting apart is exactly how a
+# category silently becomes unreachable.
 #
 # NETWORK_REDACTED_MESSAGE REPLACES a message after classification (`user_error_text`). It is written
 # to the shared asset too, and a second job that inherits that failed asset re-diagnoses the stored
@@ -53,11 +54,16 @@ from app.downloads.credentials import host_key_for_url
 # constants, imported above; each has a rule below that gives its category back, and a catalog entry
 # that gives the site's sentence.
 QUOTA_FULL_MESSAGE = "Your download storage is full — free up space, then try again."
-# A cut clip held to the space left, which is not zero: "full" would be untrue.
+# A cut clip, or an upload's converted file, held to the space left, which is not zero: "full"
+# would be untrue. Both classify by the one phrase they share.
 CLIP_TOO_BIG_MESSAGE = (
     "The clip is larger than the download storage you have left — "
     "free up space or cut a shorter clip, then try again."
 )
+CONVERTED_TOO_BIG_MESSAGE = (
+    "The converted file is larger than the download storage you have left — "
+    "free up space, then try again."
+)
 UPLOAD_MISSING_MESSAGE = "The uploaded file is missing."
 NETWORK_REDACTED_MESSAGE = "The server could not reach that address."