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.

tsconfig.json

@@ -0,0 +1,18 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "Node16",
+    "moduleResolution": "Node16",
+    "outDir": "dist",
+    "rootDir": "src",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "resolveJsonModule": true,
+    "declaration": true,
+    "sourceMap": true
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist"]
+}