curl --request POST \
  --url https://app.blogbowl.io/api/v1/newsletters/{newsletter_id}/emails \
  --header 'Content-Type: application/json' \
  --data '
{
  "subject": "<string>",
  "preview": null,
  "content_html": null,
  "content_md": null,
  "author_id": null
}
'

{
  "id": 123,
  "subject": "<string>",
  "preview": "<string>",
  "slug": "<string>",
  "status": "<string>",
  "content_html": "<string>",
  "content_json": {
    "type": "<string>",
    "content": [
      "<string>"
    ]
  },
  "author_id": 123,
  "newsletter_id": 123,
  "scheduled_at": "<string>",
  "sent_at": "<string>",
  "created_at": "<string>",
  "updated_at": "<string>"
}