use @prisma/adapter-pg with PrismaClient (required by prisma v7)

backend/src/prisma/prisma.service.ts

@@ -1,10 +1,12 @@
 import { Injectable, OnModuleInit, OnModuleDestroy } from '@nestjs/common';
 import { PrismaClient } from '@prisma/client';
+import { PrismaPg } from '@prisma/adapter-pg';
 
 @Injectable()
 export class PrismaService extends PrismaClient implements OnModuleInit, OnModuleDestroy {
   constructor() {
-    super({});
+    const adapter = new PrismaPg({ connectionString: process.env.DATABASE_URL });
+    super({ adapter });
   }
 
   async onModuleInit() {