Deprecated: get the legacy single webhook configured for a monitor

Configured notification endpoint. The signed JSON body for an HTTP attempt contains event, monitor_id, account_id, occurred_at, delivery_id, attempt, plus reason/monitor_name/monitor_url when non-empty. Since issue #165, the body also carries collapsed_transitions (integer) when a delivery superseded one or more earlier transitions for the same monitor+webhook pair — e.g. a down→up→down→up flap against a slow-to-recover subscriber can collapse to a survivor carrying collapsed_transitions: 3. The field is omitted entirely (not sent as 0) when nothing was collapsed, so existing subscribers see a byte-identical body to before this field existed.