Add CloudWatch error alarm for afterhours-ring-scheduler

The live afterhours-ring-scheduler Lambda had no error alarm. Add a
CloudWatch Errors alarm mirroring the existing HolidayRouterErrorAlarm:
AWS/Lambda Errors, Sum over one 5-min period, threshold >=1, missing
data notBreaching, paging the site-alerts SNS topic.

The orphaned Lambda-Errors-3cx-ring-group-scheduler alarm (pointing at
a renamed/absent function) is being removed separately.

template.yaml

@@ -282,6 +282,29 @@ Resources:
             Description: "Update 3CX queue at 5pm EDT weekends"
             Enabled: true
 
+  # Lambda error alarm for the ring scheduler. Mirrors the account-wide
+  # operational convention (Lambda-Errors-<fn>, threshold 1 over one 5-min
+  # period, Sum, missing=notBreaching) and pages the same site-alerts SNS topic
+  # → AWS Chatbot → Slack as the other afterhours-* functions.
+  RingSchedulerErrorAlarm:
+    Type: AWS::CloudWatch::Alarm
+    Properties:
+      AlarmName: !Sub "Lambda-Errors-${RingSchedulerFunction}"
+      AlarmDescription: "Ring scheduler Lambda reported one or more errors"
+      Namespace: AWS/Lambda
+      MetricName: Errors
+      Dimensions:
+        - Name: FunctionName
+          Value: !Ref RingSchedulerFunction
+      Statistic: Sum
+      Period: 300
+      EvaluationPeriods: 1
+      Threshold: 1
+      ComparisonOperator: GreaterThanOrEqualToThreshold
+      TreatMissingData: notBreaching
+      AlarmActions:
+        - !Sub "arn:aws:sns:${AWS::Region}:${AWS::AccountId}:site-alerts"
+
   # --- Holiday Router (repoints 3CX IVR/queue for a holiday day-shift) ---
   # Invoked with {"action": "activate"|"deactivate", "date": "<YYYY-MM-DD>"} at
   # 08:00 ET (activate) and 17:00 ET (deactivate) for each holiday date. Both