Make `add-superadmin-user` idempotent

[prophile]

Re-running add-superadmin-user with the same arguments currently produces an error:

APIError: User already exists. Use another email.
     status: "BAD_REQUEST",
       body: {
  code: "USER_ALREADY_EXISTS_USE_ANOTHER_EMAIL",
  message: "User already exists. Use another email.",
},
    headers: {},
 statusCode: 400,

This means you can't use add-superadmin-user in part of a setup script can be rerun (unlike bun install and bun run db:migrate)