Testing the previous commit against the live Loomio user list showed it would
also have renamed a member whose handle was chosen by hand (a working name
would have become the sanitized OIDC uid) — the uid is domain-shaped, so the
sanitizer produced a different, wrong target. The function is scoped to
Loomio's own auto-generated handles: the email local part with
non-alphanumerics stripped plus a random suffix. Gate on that shape.
Verified against the live user list (70 users) before building: exactly one
handle changes — the member whose username fix 422'd on every sweep — no
computed target violates Loomio's rule, and the hand-chosen handle is left
alone.