feat: add MercadoPago payments, subscription service, and webhooks

- MercadoPago PreApproval integration for recurring subscriptions
- Subscription service with caching, manual payment, payment history
- Webhook handler with HMAC-SHA256 signature verification
- Admin endpoints for subscription management and payment links
- Email notifications on payment success/failure/cancellation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/package.json

@@ -28,6 +28,7 @@
     "fast-xml-parser": "^5.3.3",
     "helmet": "^8.0.0",
     "jsonwebtoken": "^9.0.2",
+    "mercadopago": "^2.12.0",
     "node-cron": "^4.2.1",
     "node-forge": "^1.3.3",
     "nodemailer": "^8.0.2",