The frontend keyed first-run setup off a null InstanceProfile.admin, but a
null admin only means "no admin-role user exists" — which also happens on a
populated instance that has lost all its admins. Such an instance was wrongly
redirected to signup, where the new account is created as a normal user (the
first-user promotion only triggers when there are zero users), leaving the
instance permanently admin-less.
Add an explicit InstanceProfile.needs_setup derived from user count == 0, and
switch the signup redirect and host tip to use it. admin stays for display only.
96cb65320b