perf(backend): createdAtをidから取得するように & 無駄なDateインスタンスの生成を避けるように

packages/backend/src/core/SignupService.ts

@@ -120,8 +120,7 @@ export class SignupService {
 			if (exist) throw new Error(' the username is already used');
 
 			account = await transactionalEntityManager.save(new MiUser({
-				id: this.idService.genId(),
-				createdAt: new Date(),
+				id: this.idService.gen(),
 				username: username,
 				usernameLower: username.toLowerCase(),
 				host: this.utilityService.toPunyNullable(host),