Constant HOSTNAME_ID_PATTERN 

pub const HOSTNAME_ID_PATTERN: &str = "[a-z0-9]+(-[a-z0-9]+)*";

A valid tenant or project id: lowercase letters and digits, joined by single hyphens. It is subdomain safe, and never holds HOSTNAME_ID_SEPARATOR, so {tenant}--{project} splits back apart.

The sign-up form uses it as an HTML pattern, so it must stay valid under the browser’s v regex flag (no unescaped - inside []).