Download Campaign Brief

A short-lived link for the campaign's brief file: {"url": ...} with redirect=false (what the app uses — it and the API are on different origins and the API is bearer-authenticated, so a link navigation cannot carry the token), else a 302 to it. Only the document the campaign names as its brief (brief_document_id), on this account; a brief that is a note with no stored file is 404 in both forms — its text travels inline on the campaign payload. A campaign with no brief serves its draft, which the payload shows as the brief.