feat: add automated backups and Prometheus/Grafana monitoring stack (Steps 17-18)

Backups: pg_dump scripts with daily/weekly rotation and Cloudflare R2 offsite sync.
Monitoring: Prometheus, Grafana, node-exporter, cAdvisor in docker-compose; /metrics
endpoint activated via prometheus_client.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

monitoring/grafana/provisioning/datasources/datasource.yml

@@ -0,0 +1,12 @@
+# Auto-provision Prometheus as the default datasource
+# Docs: https://grafana.com/docs/grafana/latest/administration/provisioning/#datasources
+
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+    editable: true