Pure signupFormEligibility (manager override bypasses) + 4 tests.
signUpForClass now: classes.manage override + sign-up-on-behalf
(targetMemberId), and a hard form gate -- if the class has a
required_form_id the member must have a form_submissions row on file
(checked via service client, since a class manager need not hold
forms.manage). create/updateClass validate the form is in-space and
published. listUpcomingSessions exposes required_form {title,url,
satisfied}. Manage UI: published-form picker on create/edit + a 'requires
form' badge. Member UI: shows the requirement, links to the form, and
replaces Sign up with 'Complete required form' until it is on file.
Mirrors the equipment cert-gate pattern.