feat: YuKassa payment integration

- services/yukassa.js: createPayment, handleWebhook
  createPayment → создаёт платёж + сохраняет в payment_orders
  handleWebhook → activates plan + charges credits on payment.succeeded
- routes/billing.js: POST /checkout, POST /webhook (публичный)
- DB: payment_orders table
- index.js: /api/billing/webhook публичный (до auth middleware)

package.json

@@ -14,6 +14,7 @@
   "author": "",
   "license": "ISC",
   "dependencies": {
+    "@a2seven/yoo-checkout": "^1.1.4",
     "axios": "^1.16.1",
     "bull": "^4.16.5",
     "cheerio": "^1.2.0",