fix(chart): derive ES_HOST from elasticsearch values

[earayu]

Summary

• make the generated aperag-env .env derive ES_HOST from top-level elasticsearch.{ES_PROTOCOL,ES_HOST,ES_PORT} when api.env.ES_HOST is empty
• keep explicit api.env.ES_HOST as the highest-priority override

Why

During the Singapore upgrade, the container env had ES_HOST_NAME, but ApeRAG runtime reads ES_HOST from the mounted .env. With api.env.ES_HOST empty, fulltext fell back to the app default instead of the configured Elasticsearch service.

Validation

• helm template aperag deploy/aperag | sed -n '/name: aperag-env/,/---/p' | rg 'ES_HOST=' -> ES_HOST=http://es-cluster-mdit-http:9200
• helm template aperag deploy/aperag --set api.env.ES_HOST=http://custom-es:9200 | sed -n '/name: aperag-env/,/---/p' | rg 'ES_HOST=' -> ES_HOST=http://custom-es:9200
• git diff --check