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>

app/core/config.py

@@ -194,6 +194,12 @@ class Settings(BaseSettings):
     sentry_environment: str = "development"  # development, staging, production
     sentry_traces_sample_rate: float = 0.1  # 10% of transactions for performance monitoring
 
+    # =============================================================================
+    # MONITORING
+    # =============================================================================
+    enable_metrics: bool = False
+    grafana_url: str = "https://grafana.wizard.lu"
+
     # =============================================================================
     # CLOUDFLARE R2 STORAGE
     # =============================================================================