feat(classes): server actions + page guard

classes.ts: class/session CRUD (classes.manage; delete refused once
sessions/signups exist, archive instead), member signUpForClass /
cancelMySignup as validated service-client writes enforcing eligibility,
capacity vs waitlist, de-dupe and earliest-waitlist promotion on cancel,
listUpcomingSessions (service-client counts so members see spots-left
without seeing who signed up), instructor markAttendance and
completeSession. completeSession awards a class's certification via the
normal grantCertification path, so it only issues when the acting
instructor also holds certifications.grant; otherwise completion still
succeeds and reports certificatesSkipped. classes-guard mirrors the
forms/certifications guards. Barrel + API_REFERENCE updated.

lib/classes-guard.ts

@@ -0,0 +1,31 @@
+import { redirect } from 'next/navigation'
+import { createClient } from '@/lib/supabase/server'
+
+// Server-component guard for the /classes management surface. Mirrors the
+// forms/certifications guards: resolve the member, check classes.manage via
+// the same RPC the RLS uses, redirect non-managers before any data is
+// fetched. The RLS independently enforces this; the redirect is UX only.
+export async function requireClassesManagerPage() {
+  const supabase = await createClient()
+  const {
+    data: { user },
+  } = await supabase.auth.getUser()
+  if (!user) redirect('/login')
+
+  const { data: member } = await supabase
+    .from('space_members')
+    .select('id, space_id, role, user_id, display_name')
+    .eq('user_id', user.id)
+    .in('status', ['current', 'unverified', 'late'])
+    .single()
+  if (!member) redirect('/login')
+
+  const { data: allowed } = await supabase.rpc('user_has_permission', {
+    uid: user.id,
+    sid: member.space_id,
+    perm: 'classes.manage',
+  })
+  if (!allowed) redirect('/dashboard')
+
+  return { supabase, member }
+}