Create a campaign

Creates a campaign. Without template_id: a blank campaign with a single "Champion" variant at 100% traffic, answered 201. With template_id (from GET /api/v1/templates): the campaign row is created right away (archived, building: true, no variants) and answered 202 while a job copies the template content and runs the AI remix when remix.remix_all_text asks for it; the campaign unarchives itself and building flips to false when the job finishes. A failed job leaves it archived with building: false and build_failed: true, and campaign mutations answer 422 while building is true. name and cta_type default to the template's. Popups, hooks, bars and embeds are not live on creation; a landing page is live once it exists and is not archived. page_path defaults to page-{id}. Requires campaigns:write.