Create a post group

Create posts for several social accounts in one request — for example the same caption and image sent to Instagram, Threads and Facebook. Each post is validated exactly like POST /v0/posts (including per-platform rules), and media may reference stored library URLs or publicly reachable URLs; an external URL shared by several posts is imported once. The request is atomic: if any post is invalid, nothing is created. Posts with action: "queue" on the same account take consecutive queue slots. Up to 50 posts per request.