fix: use Volcengine ALB wildcard ingress paths

[yuyufei101]

Summary

• render wildcard HTTP Ingress paths for Volcengine Standard ALB prefix matching
• keep exact paths for non-slashed API/WebSocket endpoints and root
• document the Volcengine ALB pathType behavior in the Helm chart README

Verification

• helm lint deploy/helm/keystone-stack
• helm template mercury deploy/helm/keystone-stack --namespace archebase-system --set ingress.enabled=true --set-string keystone.image.tag=5c789921435f3a907ad5612c8d4dd6c9cff9b691 --set-string synapse.image.tag=eb348701e3bcbd2b8fcd4cc371dca436417c8e0c --set-string credentials.hilbertAccessKey=placeholder --set-string credentials.hilbertSecretKey=placeholder
• kubectl --context=volcano-prod -n archebase-system apply --dry-run=server -f /tmp/keystone-mercury-wildcard-ingress-rendered.yaml