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/prometheus/prometheus.yml

@@ -5,6 +5,11 @@ global:
 rule_files:
   - 'alert-rules.yml'
 
+alerting:
+  alertmanagers:
+    - static_configs:
+        - targets: ['alertmanager:9093']
+
 scrape_configs:
   - job_name: 'goodgo-api'
     metrics_path: '/metrics'