Generate a hero image for existing content

Queue one AI hero image for an article that already exists and attach it as the featured image. The article body, brief and meta are never touched, so edits made after generation are kept. Runs asynchronously: a render takes 20 to 60 seconds. Poll GET /content/{id}: progress_details.heroImageStatus goes queued then completed (image on featured_image) or failed (heroImageFailureReason says why; stop polling). Uses one image credit and one AI-image unit of the site budget, both reserved at request time and refunded exactly once if the render fails. Refuses with 409 when the article already has a hero image unless replace is true.