feat(backend): Prisma schema, Express routes, audit middleware, error handling

backend/src/lib/prisma.js

@@ -0,0 +1,7 @@
+const { PrismaClient } = require('@prisma/client');
+
+const prisma = new PrismaClient({
+  log: process.env.NODE_ENV === 'development' ? ['query', 'error'] : ['error'],
+});
+
+module.exports = prisma;