Migrate billing backend from Django to Express + Apollo Server + Prisma

Replace Python/Django/Graphene with TypeScript/Express/Apollo Server.
Same 2 endpoints (team/m2m), same JWT auth, same TigerBeetle integration.
Prisma ORM replaces Django ORM for Account/OperationCode/ServiceAccount.

src/db.ts

@@ -0,0 +1,3 @@
+import { PrismaClient } from '@prisma/client'
+
+export const prisma = new PrismaClient()