A financial services company runs a customer-facing web application on Azure App Service instances deployed in two regions: East US (primary) and West US (standby). The West US deployment is fully provisioned but should only receive user traffic if the East US region becomes unavailable. Both endpoints are exposed via public DNS names, and the application does not require sub-100ms global latency optimization. The architect must recommend a DNS-based global routing solution that directs all traffic to East US under normal conditions and automatically fails over to West US only during an outage. Which routing configuration should be recommended?