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/observability.py

@@ -515,17 +515,6 @@ external_tools = ExternalToolConfig()
 health_router = APIRouter(tags=["Health"])
 
 
-@health_router.get("/health")
-async def health_check() -> dict[str, Any]:
-    """
-    Aggregated health check endpoint.
-
-    Returns combined health status from all registered checks.
-    """
-    result = health_registry.run_all()
-    return result.to_dict()
-
-
 @health_router.get("/health/live")
 async def liveness_check() -> dict[str, str]:
     """