Previously createAdminUser only inserted a local admin_users row and
emailed the password as plaintext — the user could not sign in because
the password was never set on a Neon Auth account.
This change makes the create flow:
1. Authorize: only platform_admin can mint new admin users.
2. Create the Neon Auth user via auth.admin.createUser, falling back
to the public /sign-up/email + emailVerified=true pattern when
the caller's Neon Auth session isn't an admin (the common case
in dev — provision-admin.ts does not set neon_auth.user.role).
3. Wrap the admin_users INSERT + admin_user_brands link in a
transaction, returning an error if the local insert fails (the
Neon Auth user is left orphaned and surfaced in the message).
4. Send the welcome email best-effort, returning success/failure
info to the UI.
The CreateUserModal now shows a success state with the temp password
(copy-to-clipboard), the welcome email status, and the auth path
used. The slide-in edit panel surfaces the password via window.alert
as a defense against the dead-code new-user path.
10 new unit tests cover authorization, the admin + signup paths, the
DB-failure orphan case, and the email best-effort behavior.
Tyler