Strong backend developer for Conversational AI App

We are an AI consulting services company with several potential clients in our sales pipeline. We aim to be the single 'source of truth' for clients when it comes to creating bespoke AI automation strategies and products. For our first client, we are developing a multimodal conversational AI app with real-time chat, secure payments, session-based billing, wallet logic, transcript storage, and strong privacy controls.

We already have the frontend foundation; we now need a skilled engineer to harden the backend, integrate payments, and make the application secure, scalable, and deployable.

This role requires someone comfortable with Node/Express or FastAPI, secure payment integrations, LLM proxying, and production-grade backend architecture.

Here are the core responsibilities for our MVP:

1. Build a secure backend (Node/Express or FastAPI) with JWT auth
2. Integrate Gemini API via a server-side proxy
3. Implement secure hosted checkout + payment webhook handling
4. Build server-authoritative session lifecycle (start/extend/end)
5. Implement encrypted transcript storage and user privacy controls
6. Add monitoring, CI, and automated tests
7. Prepare the application for real-world deployment

For each point, please explain in detail:

1. What work needs to be done
2. What steps you would take
3. What technologies or patterns you would use, and
4. What major risks or edge cases we should consider
5. Estimated (please begin your application with 'Estimate') time take to complete each core responsibility

NDA required. Milestones based project. Send in your applications. Looking to find the right freelancer for long time collab.