Approve a mailbox action

Records approval of a send and returns 202 while it is pending. A send is queued for automatic retry within 15 minutes of approval; repeating approval does not extend this deadline. Accepted, completed, and notSent replays return 200. No expiry applies before approval. Only a person signed in or through OAuth may approve, and never the proposer. Non-send approvals execute synchronously and return 200 with their own execution states. The action's resolution names the approver; approving a held send publishes one mailbox.action.resolved event.