Bugbot: nulling a dangling artifact_id on a ready/preparing remux row
left it advertised — ready rows would silently fall back to serving the
source file and preparing rows hung forever (reconciliation only visits
artifact-linked rows) while consuming quota. The backfill now fails those
rows with a retryable message instead; clients re-request and get a
fresh artifact.
Also: misspell (cancelled->canceled in comments/locals) and sloglint
(literal log messages) findings from CI's changed-lines lint.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
