feat: YuKassa checkout button on /plans + API route

/plans: кнопка Подключить → fetch /api/billing/checkout → redirect to ЮKassa
app/api/billing/checkout/route.js — прокси к engine

app/api/billing/checkout/route.js

@@ -0,0 +1,15 @@
+import { NextResponse } from 'next/server';
+import { requireUser } from '@/lib/session';
+import { engine } from '@/lib/engine';
+
+export async function POST(req) {
+  const user = await requireUser();
+  if (!user) return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  try {
+    const { plan_code } = await req.json();
+    const data = await engine.call('/api/billing/checkout', {
+      userId: user.id, method: 'POST', body: { plan_code },
+    });
+    return NextResponse.json(data);
+  } catch (err) { return NextResponse.json({ error: err.message }, { status: 500 }); }
+}

app/plans/page.js

@@ -96,7 +96,17 @@ export default function PlansPage() {
                 <Link href="/register" className={`w-full text-center py-2 rounded-lg text-sm ${style.btnClass}`}>Начать бесплатно</Link>
               ) : (
                 <button className={`w-full text-center py-2 text-sm ${style.btnClass}`}
-                  onClick={() => alert('ЮKassa — скоро')}>
+                  onClick={async () => {
+                    try {
+                      const res = await fetch('/api/billing/checkout', {
+                        method: 'POST',
+                        headers: { 'Content-Type': 'application/json' },
+                        body: JSON.stringify({ plan_code: plan.code }),
+                      }).then(r => r.json());
+                      if (res.confirmationUrl) window.location.href = res.confirmationUrl;
+                      else alert(res.error || 'Ошибка создания платежа');
+                    } catch { alert('Ошибка соединения'); }
+                  }}>
                   Подключить
                 </button>
               )}