feat: add user submission history preparations

src/modules/submissions/entities/submission.entity.ts

@@ -19,6 +19,9 @@ export class Submission {
   @PrimaryGeneratedColumn()
   id!: number;
 
+  @Column({ type: 'varchar', nullable: true })
+  code!: string;
+
   @ManyToOne(() => User, { onDelete: 'CASCADE' })
   user!: User;
 
@@ -43,4 +46,7 @@ export class Submission {
 
   @CreateDateColumn()
   createdAt!: Date;
+
+  @Column({ type: 'timestamptz', nullable: true })
+  completedAt?: Date;
 }