docs: Add deployment and production setup guide

## Summary
Document how to deploy the app (e.g. Vercel + Railway, or similar) and configure production env vars.

## Scope
- [ ] Production env vars (DATABASE_URL, HUGGINGFACE_TOKEN, JWT_SECRET, CORS origins)
- [ ] Build and run commands for frontend/backend
- [ ] Optional: Docker or docker-compose for local/production
- [ ] Note on MongoDB (Atlas) and file storage for uploads in serverless