Get the secret that signs generation webhooks

Generates one image, video or voice with a model from `GET /generations/models`, or with the recommended one for a `preset`, from a prompt alone or from images. Charges the credits at once; a generation that `failed` gives them back. `credits` is the total price the user agreed to: take it from `POST /generations/quote`. A different current price answers `PRICE_CHANGED` with the new total and charges nothing. Always tell the user the model and the price and wait for their yes before calling this. To know when it ends, pass `webhook_url` (CutPro posts the generation there, signed), or read `GET /generations/{id}?wait=60`, which answers as soon as it ends. Send an `Idempotency-Key` header (any unique string, such as a UUID) to retry safely: the same key within 24 hours answers the first result and never charges twice. `image_ids` come from `POST /generations/images` or from the `output.image_id` of a ready image: none for `text`, one for `image`, two (first, last) for `frames`, and up to the model's `max_images` for `references`. A voice (`kind: speech`) reads `prompt` word for word in one of the model's `voices`, passed as `voice`; the result is an MP3.