Generate compact members-only username for new member accounts

You update Membership to Member but many accounts lack a short username, causing support friction. It generates a compact members-only username so coordinators can onboard members same day.

Overview

Missing or inconsistent usernames slow membership onboarding and frustrate coordinators; this flow auto-generates a compact members-only handle and writes it to the account record. That removes manual edits and gets members searchable and ready for onboarding same day.