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/docker-compose.monitoring.yml

@@ -55,6 +55,20 @@ services:
     ports:
       - "9121:9121"
 
+  alertmanager:
+    image: prom/alertmanager:v0.27.0
+    container_name: nexus-alertmanager
+    restart: unless-stopped
+    ports:
+      - "9093:9093"
+    volumes:
+      - ./alertmanager:/etc/alertmanager
+      - alertmanager-data:/alertmanager
+    command:
+      - '--config.file=/etc/alertmanager/alertmanager.yml'
+      - '--storage.path=/alertmanager'
+
 volumes:
   prometheus-data:
   grafana-data:
+  alertmanager-data: