fix: split Keystone ALB health check

[yuyufei101]

Summary

• split Keystone HTTP paths into a separate ALB Ingress
• configure Keystone ALB health check to /api/v1/health
• keep Synapse root routing on its own Ingress with the default root health check

Validation

• helm lint deploy/helm/keystone-stack
• helm template mercury deploy/helm/keystone-stack --namespace archebase-system --set-string keystone.image.tag=b4ece5364a5584749759af85c2e92e757671d072 --set-string synapse.image.tag=eb348701e3bcbd2b8fcd4cc371dca436417c8e0c --set-string credentials.hilbertAccessKey=placeholder --set-string credentials.hilbertSecretKey=placeholder