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.

prisma/schema.prisma

@@ -0,0 +1,43 @@
+generator client {
+  provider = "prisma-client-js"
+}
+
+datasource db {
+  provider = "postgresql"
+  url      = env("BILLING_DATABASE_URL")
+}
+
+enum AccountType {
+  USER
+  SERVICE
+}
+
+model Account {
+  uuid        String      @id @default(uuid()) @db.Uuid
+  name        String      @db.VarChar(255)
+  accountType AccountType @default(USER) @map("account_type")
+  description String?
+  createdAt   DateTime    @default(now()) @map("created_at")
+
+  serviceInfo ServiceAccount?
+
+  @@map("billing_app_account")
+}
+
+model OperationCode {
+  id          Int      @id @default(autoincrement())
+  code        Int      @unique
+  name        String   @unique @db.VarChar(255)
+  description String?
+  createdAt   DateTime @default(now()) @map("created_at")
+
+  @@map("billing_app_operationcode")
+}
+
+model ServiceAccount {
+  accountUuid String  @id @map("account_id") @db.Uuid
+  slug        String  @unique @db.VarChar(50)
+  account     Account @relation(fields: [accountUuid], references: [uuid], onDelete: Cascade)
+
+  @@map("billing_app_serviceaccount")
+}