Function create_first_admin 

pub async fn create_first_admin(
    pool: &PgPool,
    username: &str,
    display_name: &str,
    password: &str,
) -> AppResult<User>

Creates the bootstrap administrator if the database contains no users. First start without any account: creates the administrator (the “create the administrator” form of the login page). Concurrent attempts are serialised with an advisory lock; once an account exists it fails.