feat: enable Alertmanager with Slack receiver in kube-prometheus-stack HelmRelease

- Set alertmanager.enabled: true (was false)
- Add Slack receiver config pointing to gatus-slack-webhook secret URL
- Add alertmanagerSpec.secrets to mount gatus-slack-webhook into Alertmanager pod
- Route: group by alertname+namespace, 30s wait, 5m interval, 4h repeat
- Channel: #api-alerts with summary title and description text

Closes leeworks-agents/api-company#210

flux/monitoring/helmrelease.yaml

@@ -85,7 +85,25 @@ spec:
                 action: replace
                 target_label: kubernetes_pod_name
     alertmanager:
-      enabled: false  # Enable when alert routing is configured
+      enabled: true
+      config:
+        global:
+          slack_api_url_file: /etc/alertmanager/secrets/gatus-slack-webhook/url
+        route:
+          receiver: slack-api-alerts
+          group_by: [alertname, namespace]
+          group_wait: 30s
+          group_interval: 5m
+          repeat_interval: 4h
+        receivers:
+          - name: slack-api-alerts
+            slack_configs:
+              - channel: '#api-alerts'
+                title: '{{ .CommonAnnotations.summary }}'
+                text: '{{ range .Alerts }}{{ .Annotations.description }}{{ end }}'
+      alertmanagerSpec:
+        secrets:
+          - gatus-slack-webhook
     kubeStateMetrics:
       enabled: true
     nodeExporter: