MERN Backend API Integration

I’ve already built the front-end of a mobile app and now need a solid MERN backend that centres on API integration. The core job is to create secure, well-documented endpoints in Node.js / Express that will:

• Connect to several third-party services (think mapping, notifications, social sign-in, etc.).
• Expose and consume our own internal microservices so the app can scale without tight coupling.
• Process payments through a leading gateway, handling tokenisation, webhooks and error states.

All data should persist in a NoSQL store—MongoDB fits naturally here—so please design the models, indices and validation accordingly.

Deliverables
1. REST (or GraphQL if you prefer) endpoints covering the integrations above.
2. Connection logic and middleware for each external service, with clear environment-based config.
3. MongoDB schemas plus seed scripts for local testing.
4. Postman (or similar) collection and a brief README so my team can hit the ground running.

Clean, modular code, meaningful commits and succinct inline comments are non-negotiable. If this sounds like your wheelhouse, let’s get started.