feat(profile): add user profile management with first and last name fields

backend/src/users/users.service.ts

@@ -16,4 +16,8 @@ export class UsersService {
   create(data: { username: string; email: string; passwordHash: string }) {
     return this.prisma.user.create({ data });
   }
+
+  updateProfile(id: number, data: { firstName?: string; lastName?: string; email?: string }) {
+    return this.prisma.user.update({ where: { id }, data });
+  }
 }