Generate an image from a text prompt and get back a URL hosted by this Forem.

The image follows this Forem's cover image style and, unless aspect_ratio is given, is sized for its cover images. Use the URL in body_markdown or as an article's main_image, just like an uploaded image.

Limits:

• prompt is required and may be up to 1000 characters.
• Rate limited per user to a configurable number of generations per configurable window (by default 3 per day). Every attempt counts, including failed ones. Exceeding it returns 429 with a Retry-After header. Forem admins can change both values, or set the limit to 0 to disable this endpoint.
• Returns 503 if AI image generation is not configured on this Forem.