Add API access logging + throttling (audit Day 3: M-18)

Implicit HTTP API: access logging to /aws/apigateway/payments-dashboard (90d) +
default route throttling (100 rps / 50 burst) via Globals.HttpApi.

template.yaml

@@ -12,8 +12,22 @@ Globals:
     Environment:
       Variables:
         TABLE_NAME: !Ref DashboardTable
+  # Access logging + default throttling on the implicit HTTP API (audit M-18).
+  HttpApi:
+    AccessLogSettings:
+      DestinationArn: !GetAtt ApiAccessLogGroup.Arn
+      Format: '{"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"}'
+    DefaultRouteSettings:
+      ThrottlingBurstLimit: 50
+      ThrottlingRateLimit: 100
 
 Resources:
+  ApiAccessLogGroup:
+    Type: AWS::Logs::LogGroup
+    Properties:
+      LogGroupName: /aws/apigateway/payments-dashboard
+      RetentionInDays: 90
+
   # VPC with private subnet + NAT Gateway for static outbound IP
   Vpc:
     Type: AWS::EC2::VPC