feat(monitoring): add Alertmanager with alert rules

- docker-compose.monitoring.yml: added alertmanager service (port 9093)
- prometheus.yml: alerting config + rule_files entry
- alerts.yml: 5 alert rules (PostgreSQLDown, RedisDown, HighDiskUsage,
  HighMemoryUsage, NodeDown)
- alertmanager.yml: SMTP + webhook receiver, inhibit rules

docker/alertmanager/alertmanager.yml

@@ -0,0 +1,34 @@
+global:
+  smtp_smarthost: 'localhost:587'
+  smtp_from: 'alerts@nexus.local'
+  smtp_require_tls: false
+
+route:
+  group_by: ['alertname', 'severity']
+  group_wait: 10s
+  group_interval: 10s
+  repeat_interval: 1h
+  receiver: 'default'
+
+receivers:
+  - name: 'default'
+    email_configs:
+      - to: 'admin@nexus.local'
+        subject: 'Nexus Alert: {{ .GroupLabels.alertname }}'
+        body: |
+          {{ range .Alerts }}
+          Alert: {{ .Annotations.summary }}
+          Description: {{ .Annotations.description }}
+          Severity: {{ .Labels.severity }}
+          Time: {{ .StartsAt }}
+          {{ end }}
+    webhook_configs:
+      - url: 'http://localhost:5001/pos/api/notifications/webhook'
+        send_resolved: true
+
+inhibit_rules:
+  - source_match:
+      severity: 'critical'
+    target_match:
+      severity: 'warning'
+    equal: ['alertname']