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-lock.json

@@ -9,6 +9,7 @@
       "version": "1.0.0",
       "license": "ISC",
       "dependencies": {
+        "@a2seven/yoo-checkout": "^1.1.4",
         "axios": "^1.16.1",
         "bull": "^4.16.5",
         "cheerio": "^1.2.0",
@@ -21,6 +22,29 @@
         "sharp": "^0.34.5"
       }
     },
+    "node_modules/@a2seven/yoo-checkout": {
+      "version": "1.1.4",
+      "resolved": "https://registry.npmjs.org/@a2seven/yoo-checkout/-/yoo-checkout-1.1.4.tgz",
+      "integrity": "sha512-Xn8E6fJKVUpSqTTEgigoi0Woyvmc1/UuHukwpQUTEtxD2l4skTEeqPlhd0J4xum6lQ/0eGzeJP+/MgpkE9Ak9g==",
+      "license": "MIT",
+      "dependencies": {
+        "@types/axios": "^0.14.0",
+        "axios": "^0.21.1",
+        "uuid": "^8.3.2"
+      },
+      "engines": {
+        "node": ">=12"
+      }
+    },
+    "node_modules/@a2seven/yoo-checkout/node_modules/axios": {
+      "version": "0.21.4",
+      "resolved": "https://registry.npmjs.org/axios/-/axios-0.21.4.tgz",
+      "integrity": "sha512-ut5vewkiu8jjGBdqpM44XxjuCjq9LAKeHVmoVfHVzy8eHgxxq8SbAVQNovDA8mVi05kP0Ea/n/UzcSHcTJQfNg==",
+      "license": "MIT",
+      "dependencies": {
+        "follow-redirects": "^1.14.0"
+      }
+    },
     "node_modules/@emnapi/runtime": {
       "version": "1.10.0",
       "resolved": "https://registry.npmjs.org/@emnapi/runtime/-/runtime-1.10.0.tgz",
@@ -580,6 +604,16 @@
         "win32"
       ]
     },
+    "node_modules/@types/axios": {
+      "version": "0.14.4",
+      "resolved": "https://registry.npmjs.org/@types/axios/-/axios-0.14.4.tgz",
+      "integrity": "sha512-9JgOaunvQdsQ/qW2OPmE5+hCeUB52lQSolecrFrthct55QekhmXEwT203s20RL+UHtCQc15y3VXpby9E7Kkh/g==",
+      "deprecated": "This is a stub types definition. axios provides its own type definitions, so you do not need this installed.",
+      "license": "MIT",
+      "dependencies": {
+        "axios": "*"
+      }
+    },
     "node_modules/accepts": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/accepts/-/accepts-2.0.0.tgz",