Function handle_viewer_pair 

async fn handle_viewer_pair(
    __arg0: State<AppState>,
    peer: Option<Extension<ConnectInfo<SocketAddr>>>,
    body: Result<Json<PairBody>, JsonRejection>,
) -> Response

POST /agentmux/viewer/pair: {code, device_name, device_key?} for {token, device_id, hostname, channel, version, install_id?}; 401 for an unknown or expired code, 429 after too many wrong ones.