fix: serve Keystone root health for ALB

[yuyufei101]

Summary

• serve Keystone HTTP root path as the existing health response for ALB backend probes
• revert the split HTTP Ingress back to a single Ingress to avoid VKE host/path conflicts during Helm upgrade
• update the Helm chart README routing note

Validation

• go test ./...
• helm lint deploy/helm/keystone-stack
• helm template mercury deploy/helm/keystone-stack --namespace archebase-system with Keystone/Synapse tags and Hilbert placeholders