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/models/Announcement.ts

@@ -12,12 +12,6 @@ export class MiAnnouncement {
 	@PrimaryColumn(id())
 	public id: string;
 
-	@Index()
-	@Column('timestamp with time zone', {
-		comment: 'The created date of the Announcement.',
-	})
-	public createdAt: Date;
-
 	@Column('timestamp with time zone', {
 		comment: 'The updated date of the Announcement.',
 		nullable: true,