INFRA-28: add HTTP API access logging and throttling (audit M-18) (#98)

Add AccessLogSettings on the implicit HTTP API stage pointing at a new
/aws/apigateway/afterhours-shift-manager log group with 90-day retention,
plus DefaultRouteSettings throttling (100 rps / 50 burst). Mirrors the
M-18 pattern landed on payments-dashboard.

template.yaml

@@ -21,8 +21,22 @@ Globals:
     MemorySize: 1024
     Architectures:
       - arm64
+  # 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/afterhours-shift-manager
+      RetentionInDays: 90
+
   # --- Shared Lambda Layer ---
   SharedLayer:
     Type: AWS::Serverless::LayerVersion