Align backend dependencies

src/db.ts

@@ -1,3 +1,11 @@
 import { PrismaClient } from '@prisma/client'
+import { PrismaPg } from '@prisma/adapter-pg'
 
-export const prisma = new PrismaClient()
+const connectionString = process.env.BILLING_DATABASE_URL
+if (!connectionString) {
+  throw new Error('BILLING_DATABASE_URL is required')
+}
+
+const adapter = new PrismaPg({ connectionString })
+
+export const prisma = new PrismaClient({ adapter })