refactor: replace UserDingMapping with StudentDingMapping entity

apps/server/src/entities/student-ding-mapping.entity.ts

@@ -0,0 +1,24 @@
+import {
+  Entity, PrimaryGeneratedColumn, Column, CreateDateColumn,
+  ManyToOne, JoinColumn,
+} from 'typeorm';
+import { Student } from './student.entity';
+
+@Entity('student_ding_mapping')
+export class StudentDingMapping {
+  @PrimaryGeneratedColumn()
+  id: number;
+
+  @Column({ name: 'ding_user_id', length: 100, unique: true })
+  dingUserId: string;
+
+  @Column({ name: 'student_id', type: 'integer', unique: true })
+  studentId: number;
+
+  @ManyToOne(() => Student, { onDelete: 'CASCADE' })
+  @JoinColumn({ name: 'student_id' })
+  student: Student;
+
+  @CreateDateColumn({ name: 'created_at' })
+  createdAt: Date;
+}