feat(api): add access logging and throttling to webhook HTTP API (#46)

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)

lib/constructs/slack-handler.ts

@@ -138,6 +138,33 @@ export class SlackHandlerConstruct extends Construct {
       });
     }
 
+    // ── Access logging + throttling (audit M-18) ──────────────────────────────
+    const defaultStage = this.api.defaultStage!.node.defaultChild as apigatewayv2.CfnStage;
+    defaultStage.addPropertyOverride('DefaultRouteSettings', {
+      ThrottlingBurstLimit: 50,
+      ThrottlingRateLimit: 100,
+    });
+
+    const apiAccessLogGroup = new logs.LogGroup(this, 'ApiAccessLogGroup', {
+      logGroupName: '/aws/apigateway/seahaven-slack-webhook',
+      retention: logs.RetentionDays.THREE_MONTHS,
+      removalPolicy: cdk.RemovalPolicy.DESTROY,
+    });
+    defaultStage.addPropertyOverride('AccessLogSettings', {
+      DestinationArn: apiAccessLogGroup.logGroupArn,
+      Format: JSON.stringify({
+        requestId: '$context.requestId',
+        ip: '$context.identity.sourceIp',
+        requestTime: '$context.requestTime',
+        method: '$context.httpMethod',
+        routeKey: '$context.routeKey',
+        status: '$context.status',
+        protocol: '$context.protocol',
+        responseLength: '$context.responseLength',
+        integrationError: '$context.integrationErrorMessage',
+      }),
+    });
+
     // ── Custom domain: bot.seahaven.com ───────────────────────────────────────
     const certificate = acm.Certificate.fromCertificateArn(
       this, 'WildcardCert', props.wildcardCertArn,