Approve a post that is waiting for approval

Approves the current step of the post's approval workflow, on behalf of the user who owns this API key. That user must be a listed approver for the workflow's CURRENT step — steps are approved in order, and a key holder who is only an approver on a later step gets forbidden until earlier steps clear. If this is the last step, the post is finalized immediately: it moves to scheduled (future schedule_at) or posting (due now or in the past). If more steps remain, the post stays in_approval and the next step's approvers are notified. Check approval_status on GET /posts/{id} first — only a post with status in_approval can be approved.