Adds an access log group (/aws/apigateway/seahaven-slack-webhook, 90-day
retention) with JSON access-log format and DefaultRouteSettings throttling
(rate 2 rps, burst 5) on the seahaven-slack-webhook HTTP API default stage,
applied via CfnStage property overrides. Matches the pattern landed on
seahaven-door-unlock-api.
Refs INFRA-29 (AWS audit M-18)