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']

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:

docker/prometheus/alerts.yml

@@ -0,0 +1,47 @@
+groups:
+  - name: nexus-alerts
+    rules:
+      - alert: PostgreSQLDown
+        expr: pg_up == 0
+        for: 1m
+        labels:
+          severity: critical
+        annotations:
+          summary: "PostgreSQL is down"
+          description: "PostgreSQL has been down for more than 1 minute."
+
+      - alert: RedisDown
+        expr: redis_up == 0
+        for: 1m
+        labels:
+          severity: critical
+        annotations:
+          summary: "Redis is down"
+          description: "Redis has been down for more than 1 minute."
+
+      - alert: HighDiskUsage
+        expr: (node_filesystem_avail_bytes / node_filesystem_size_bytes) * 100 < 10
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Disk usage is high"
+          description: "Disk usage is above 90% on {{ $labels.device }}."
+
+      - alert: HighMemoryUsage
+        expr: (node_memory_MemTotal_bytes - node_memory_MemAvailable_bytes) / node_memory_MemTotal_bytes * 100 > 85
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "Memory usage is high"
+          description: "Memory usage is above 85%."
+
+      - alert: NodeDown
+        expr: up{job="node"} == 0
+        for: 2m
+        labels:
+          severity: critical
+        annotations:
+          summary: "Node exporter is down"
+          description: "Node exporter has been down for more than 2 minutes."

docker/prometheus/prometheus.yml

@@ -2,6 +2,16 @@ global:
   scrape_interval: 15s
   evaluation_interval: 15s
 
+# Alertmanager configuration
+alerting:
+  alertmanagers:
+    - static_configs:
+        - targets: ['alertmanager:9093']
+
+# Load rules once and periodically evaluate them
+rule_files:
+  - 'alerts.yml'
+
 scrape_configs:
   - job_name: 'prometheus'
     static_configs: