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/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."