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

apps/server/src/bills/bills.service.ts

@@ -1,6 +1,6 @@
 import { Injectable, NotFoundException } from '@nestjs/common';
 import { InjectRepository } from '@nestjs/typeorm';
-import { Repository, DataSource } from 'typeorm';
+import { Repository, In, DataSource } from 'typeorm';
 import { Bill } from '../entities/bill.entity';
 import { BillItem } from '../entities/bill-item.entity';
 import { RoomExpense } from '../entities/room-expense.entity';
@@ -177,10 +177,19 @@ export class BillsService {
       });
     }
 
+
     // 合并所有涉及的学生
     const allStudentIds = new Set([...studentBillData.keys(), ...personalMap.keys()]);
-
     // 生成账单
+    // Batch-load student departmentIds
+    const studentIdsArr = [...allStudentIds];
+    const studentDeptMap = new Map<number, number>();
+    if (studentIdsArr.length > 0) {
+      const studentsData: { id: number; department_id: number | null }[] = await this.dataSource.query(
+        `SELECT id, department_id FROM students WHERE id IN (${studentIdsArr.join(',')})`
+      );
+      for (const s of studentsData) if (s.department_id != null) studentDeptMap.set(s.id, s.department_id);
+    }
     const bills: Bill[] = [];
     for (const studentId of allStudentIds) {
       const shared = studentBillData.get(studentId)?.shared || 0;
@@ -195,6 +204,7 @@ export class BillsService {
         personalAmount: personal,
         totalAmount: total,
         status: 'draft',
+        departmentId: studentDeptMap.get(studentId),
       });
       const savedBill = await this.billRepo.save(bill);