Create `backend/` as its own package in an npm workspace, add Prisma, and connect it to a local PostgreSQL database. **Done when:** - [ ] `backend/` is set up with TypeScript and Prisma, and installs from the repo root with `npm install` - [ ] The existing `backend/src/schema.graphql` is kept - [ ] The PostgreSQL major version is chosen and written in the README - [ ] `backend/.env.example` shows the required variables, and `.env` is ignored by git - [ ] The README explains how to install PostgreSQL locally on Mac and Windows