feat: auto-populate department_id on create + seed default campus with backfill

apps/server/src/departments/seed.ts

@@ -0,0 +1,23 @@
+import { DataSource } from 'typeorm';
+
+export async function seedDefaultCampus(dataSource: DataSource) {
+  const deptRepo = dataSource.getRepository('departments');
+  const userDeptRepo = dataSource.getRepository('user_departments');
+
+  const existing = await deptRepo.count();
+  if (existing > 0) { console.log('Departments exist, skip seed'); return; }
+
+  const campus = await deptRepo.save({ name: '主校区', type: 'campus', sortOrder: 0 });
+
+  const tables = ['students','rooms','classrooms','class_schedules','attendance_records','room_expenses','personal_expenses','occupancies','bills','deposits','deposit_installments','classroom_rentals'];
+  for (const table of tables) {
+    await dataSource.query(`UPDATE ${table} SET department_id = ? WHERE department_id IS NULL`, [campus.id]);
+  }
+
+  const users = await dataSource.query('SELECT id FROM users');
+  for (const user of users) {
+    await userDeptRepo.save({ userId: user.id, departmentId: campus.id, isDefault: true });
+  }
+
+  console.log('Seed: default campus created, data backfilled');
+}