feat(monitoring): add comprehensive alerting rules, Alertmanager, and DR validation

Expand production monitoring with full alert coverage for database connections,
Redis memory/connections, container resources, disk usage, service health, and
backup integrity. Add Alertmanager service with Slack routing for critical and
warning alerts, and add automated backup verification to the pg-backup cron
schedule. Update runbook with DR validation procedures and quarterly checklist.

- Expand Prometheus alert rules from 4 to 24 alerts across 7 groups
- Add Alertmanager container (prom/alertmanager:v0.27.0) with Slack routing
- Configure inhibition rules (critical suppresses warning for same service)
- Schedule automated backup verification at 04:00 UTC daily
- Add Alertmanager datasource to Grafana provisioning
- Update runbook with Section 9: DR Validation (automated + manual procedures)
- Add SLACK_WEBHOOK_URL and Grafana vars to .env.example

Co-Authored-By: Paperclip <noreply@paperclip.ing>

monitoring/grafana/provisioning/datasources/datasource.yml

@@ -21,3 +21,12 @@ datasources:
           matcherRegex: 'correlationId":"([^"]+)'
           name: correlationId
           url: '$${__value.raw}'
+
+  - name: Alertmanager
+    uid: alertmanager
+    type: alertmanager
+    access: proxy
+    url: http://alertmanager:9093
+    editable: true
+    jsonData:
+      implementation: prometheus