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.

app/(app)/classes/manage/page.tsx

@@ -6,10 +6,10 @@ export const dynamic = 'force-dynamic'
 export default async function ClassesManagePage() {
   const { supabase, member } = await requireClassesManagerPage()
 
-  const [{ data: classes }, { data: sessions }, { data: certs }] = await Promise.all([
+  const [{ data: classes }, { data: sessions }, { data: certs }, { data: forms }] = await Promise.all([
     supabase
       .from('classes')
-      .select('id, title, description, payment_link, capacity, is_active, grants_certification_id, created_at, updated_at')
+      .select('id, title, description, payment_link, capacity, is_active, grants_certification_id, required_form_id, created_at, updated_at')
       .eq('space_id', member.space_id)
       .order('is_active', { ascending: false })
       .order('title', { ascending: true }),
@@ -24,6 +24,12 @@ export default async function ClassesManagePage() {
       .eq('space_id', member.space_id)
       .eq('is_active', true)
       .order('name', { ascending: true }),
+    supabase
+      .from('forms')
+      .select('id, title, kind')
+      .eq('space_id', member.space_id)
+      .eq('status', 'published')
+      .order('title', { ascending: true }),
   ])
 
   return (
@@ -31,6 +37,7 @@ export default async function ClassesManagePage() {
       initialClasses={classes ?? []}
       initialSessions={sessions ?? []}
       certs={certs ?? []}
+      forms={forms ?? []}
     />
   )
 }