feat(classes): optional required-form gate at signup (waiver-on-file)

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.

lib/classes-logic.ts

@@ -63,6 +63,21 @@ export function canSignUp(input: {
   return { ok: true }
 }
 
+// A class may require a form (waiver/intake) on file. Mirrors the equipment
+// required-certification gate: the requirement is bypassed for a manager
+// acting with override (e.g. signing a member up on their behalf). "On file"
+// means the member has at least one completed submission of that form.
+export function signupFormEligibility(input: {
+  requiresForm: boolean
+  memberHasForm: boolean
+  managerOverride: boolean
+}): SignupEligibility {
+  if (input.requiresForm && !input.memberHasForm && !input.managerOverride) {
+    return { ok: false, reason: 'This class requires a form to be completed before you can sign up.' }
+  }
+  return { ok: true }
+}
+
 // When a registered member cancels and a seat frees, the earliest-signed-up
 // waitlisted member is promoted. Returns that signup id, or null if there is
 // no free seat or no one waiting.