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/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: