Add messages-present alarms on async-invoke DLQs (#48)

The two async-invoke OnFailure DLQs (payments-processPaymentCsv-async-dlq
and payments-processPayrollEmail-async-dlq) had no CloudWatch alarm, so a
failed async invocation could sit in the DLQ unnoticed. Add a
messages-present alarm for each, mirroring PayrollBatchDLQAlarm exactly:
AWS/SQS ApproximateNumberOfMessagesVisible, Maximum, threshold > 0,
Period 300, EvaluationPeriods 1, TreatMissingData notBreaching, ALARM-only
to the site-alerts SNS topic.

template.yaml

@@ -759,6 +759,48 @@ Resources:
       AlarmActions:
         - !Sub arn:aws:sns:${AWS::Region}:${AWS::AccountId}:site-alerts
 
+  # Messages-present alarms on the async-invoke OnFailure DLQs, mirroring
+  # PayrollBatchDLQAlarm. Threshold > 0 on the visible-message count, ALARM-only.
+  ProcessPaymentCsvDLQAlarm:
+    Type: AWS::CloudWatch::Alarm
+    Properties:
+      AlarmName: payments-processPaymentCsv-async-dlq-messages
+      AlarmDescription: Failed processPaymentCsv async invocations landed in the DLQ
+      Namespace: AWS/SQS
+      MetricName: ApproximateNumberOfMessagesVisible
+      Dimensions:
+        - Name: QueueName
+          Value: !GetAtt ProcessPaymentCsvDLQ.QueueName
+      Statistic: Maximum
+      Period: 300
+      EvaluationPeriods: 1
+      Threshold: 0
+      ComparisonOperator: GreaterThanThreshold
+      TreatMissingData: notBreaching
+      # ALARM-only notification by convention — no OK/recovery action
+      AlarmActions:
+        - !Sub arn:aws:sns:${AWS::Region}:${AWS::AccountId}:site-alerts
+
+  ProcessPayrollEmailDLQAlarm:
+    Type: AWS::CloudWatch::Alarm
+    Properties:
+      AlarmName: payments-processPayrollEmail-async-dlq-messages
+      AlarmDescription: Failed processPayrollEmail async invocations landed in the DLQ
+      Namespace: AWS/SQS
+      MetricName: ApproximateNumberOfMessagesVisible
+      Dimensions:
+        - Name: QueueName
+          Value: !GetAtt ProcessPayrollEmailDLQ.QueueName
+      Statistic: Maximum
+      Period: 300
+      EvaluationPeriods: 1
+      Threshold: 0
+      ComparisonOperator: GreaterThanThreshold
+      TreatMissingData: notBreaching
+      # ALARM-only notification by convention — no OK/recovery action
+      AlarmActions:
+        - !Sub arn:aws:sns:${AWS::Region}:${AWS::AccountId}:site-alerts
+
   ProcessPayrollEmailLogGroup:
     Type: AWS::Logs::LogGroup
     Properties: