CompleteUpload writes it (SQL NULL when there was none, never the JSON text 'null':
the page claims a second opinion only if the column is non-NULL); GetUpload reads it,
List does not. Mutation-checked.
domain.go and the store are one commit: the store tests need the new field.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>