module-level-env-reads-with-dev-fallback-bypass-runtime-config

cube_client.py read CUBEJS_API_SECRET at import time with a hardcoded dev fallback string. Module-level reads don’t respect runtime env changes and the dev fallback (a publicly known default) would be used in production if the env var wasn’t set. Refactored to a lazy _cube_api_secret() function that raises on empty secret and refuses to sign with the dev key when RDASH_ENV=production.