Function init_db 

pub async fn init_db(
    database_url: &str,
) -> Result<DatabaseConnection, DurableError>

Initialize durable: connect to Postgres and run migrations only.

Does not start heartbeat or recovery loops. Use this for tests, migrations-only scripts, or when you manage the Executor yourself.

let db = durable::init_db("postgres://localhost/mydb").await?;