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 }); }
+}