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>

main.py

@@ -237,6 +237,11 @@ else:
 # Include API router (JSON endpoints at /api/*)
 app.include_router(api_router, prefix="/api")
 
+# Include observability endpoints (/metrics, /health/live, /health/ready, /health/tools)
+from app.core.observability import health_router
+
+app.include_router(health_router)
+
 # ============================================================================
 # FAVICON ROUTES (Must be registered BEFORE page routers)
 # ============================================================================