fix(prod): use in-cluster Hilbert service

[yuyufei101]

Summary

• route production Keystone-to-Hilbert traffic through the in-cluster backend Service
• document the production internal Hilbert endpoint

Validation

• helm lint deploy/helm/keystone-stack with production release values
• rendered KEYSTONE_HILBERT_BASE_URL as http://hilbert-be-backend:8080/hilbert-be-backend
• verified production Service archebase-system/hilbert-be-backend:8080 has ready endpoints