refactor: API口径统一 — 统计函数/员工列表/前端服务层

后端:
- /dashboard/compliance-score 改用 getHealthCheck，与体检诊断同口径
- 新增 /employees/list 端点（不分页，供各页面下拉选择）
- /employees/all-lite 增加 position 字段和 status 筛选参数
- getHealthCheck 未签合同改为查无合同记录的员工（非UNSIGNED类型）

前端:
- 新建 lib/api-services.ts 统一 API 服务层
- employeeApi/rosterApi/dashboardApi/attendanceApi 统一封装
- 6个页面迁移到统一 API 调用:
  - Attendance: employeeApi.list + rosterApi.departments
  - Termination: rosterApi.list + rosterApi.departments
  - Money: rosterApi.list + rosterApi.departments
  - PortalQRModal: employeeApi.list
  - AIAssistant: 3处 employeeApi.list 替代 /roster?pageSize=999
  - Contracts: rosterApi.departments
  - Dashboard: rosterApi.expiringContracts + dashboardApi.healthCheck/workforceStats

frontend/src/pages/Compensation.tsx

@@ -2,6 +2,7 @@ import { useState, useMemo } from 'react'
 import { useQuery } from '@tanstack/react-query'
 import { Calculator, Info, AlertCircle } from 'lucide-react'
 import api from '../lib/api'
+import { rosterApi } from '../lib/api-services'
 import Card from '../components/ui/Card'
 import Button from '../components/ui/Button'
 import { Input, Label, Select } from '../components/ui/Input'