Function secret_key_to_env_var 

pub(crate) fn secret_key_to_env_var(key: &str, prefix: Option<&str>) -> String

Transform a secret key name into an environment variable name.

Rules: uppercase, hyphens and dots become underscores, strip non-alphanumeric except underscores. With prefix “MYAPP”: “api-key” -> “MYAPP_API_KEY”.