feat: 已离职/解聘员工可重新入职

1. 后端新增 rehireEmployee 函数和 POST /employees/:id/rehire 接口
2. 校验：仅已离职员工可重新入职，新入职日期须晚于上次离职日期
3. 复用员工已有基本信息，只需填写新入职日期和劳动合同
4. 前端花名册已离职员工显示「重新入职」按钮+弹窗（RehireModal）

frontend/src/pages/Roster.tsx

@@ -1,6 +1,6 @@
 import { useState, useRef } from 'react'
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
-import { Users, FileText, AlertTriangle, Calendar, GraduationCap, TrendingUp, Scale, X, Plus, Paperclip, Trash2, Printer, Calculator, Shield, Info, Check, Eye, Download, UserX } from 'lucide-react'
+import { Users, FileText, AlertTriangle, Calendar, GraduationCap, TrendingUp, Scale, X, Plus, Paperclip, Trash2, Printer, Calculator, Shield, Info, Check, Eye, Download, UserX, UserPlus } from 'lucide-react'
 import api from '../lib/api'
 import Card from '../components/ui/Card'
 import Button from '../components/ui/Button'
@@ -21,6 +21,8 @@ export default function Roster() {
   const [showAddModal, setShowAddModal] = useState(false)
   const [showResignModal, setShowResignModal] = useState(false)
   const [resignEmployee, setResignEmployee] = useState<any>(null)
+  const [showRehireModal, setShowRehireModal] = useState(false)
+  const [rehireEmployee, setRehireEmployee] = useState<any>(null)
   const [page, setPage] = useState(1)
   const [pageSize, setPageSize] = useState(10)
 
@@ -59,6 +61,16 @@ export default function Roster() {
     },
   })
 
+  const rehireMutation = useMutation({
+    mutationFn: (data: any) => api.post(`/employees/${rehireEmployee?.id}/rehire`, data),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['roster'] })
+      queryClient.invalidateQueries({ queryKey: ['dashboard'] })
+      setShowRehireModal(false)
+      setRehireEmployee(null)
+    },
+  })
+
   const filtered = employees?.filter((e: any) =>
     !search || e.name.includes(search) || e.department.includes(search)
   ) || []
@@ -181,6 +193,18 @@ export default function Roster() {
                           </button>
                         </div>
                       )}
+                      {e.status === 'RESIGNED' && (
+                        <button
+                          className="text-xs text-primary hover:text-primary/80 flex items-center gap-0.5"
+                          onClick={(ev) => {
+                            ev.stopPropagation()
+                            setRehireEmployee(e)
+                            setShowRehireModal(true)
+                          }}
+                        >
+                          <UserPlus className="w-3.5 h-3.5" />重新入职
+                        </button>
+                      )}
                     </td>
                   </tr>
                 ))}
@@ -208,6 +232,16 @@ export default function Roster() {
           error={resignMutation.error as any}
         />
       )}
+
+      {showRehireModal && rehireEmployee && (
+        <RehireModal
+          employee={rehireEmployee}
+          onClose={() => { setShowRehireModal(false); setRehireEmployee(null) }}
+          onSubmit={(data) => rehireMutation.mutate(data)}
+          loading={rehireMutation.isPending}
+          error={rehireMutation.error as any}
+        />
+      )}
     </div>
   )
 }
@@ -748,6 +782,140 @@ function ResignModal({ employee, onClose, onSubmit, loading, error }: {
   )
 }
 
+function RehireModal({ employee, onClose, onSubmit, loading, error }: {
+  employee: any
+  onClose: () => void
+  onSubmit: (data: any) => void
+  loading: boolean
+  error: any
+}) {
+  const [form, setForm] = useState({
+    hireDate: new Date().toISOString().slice(0, 10),
+    isPregnant: false,
+    isInMedicalPeriod: false,
+    isWorkInjured: false,
+    contractType: 'FIXED' as 'FIXED' | 'UNFIXED' | 'UNSIGNED',
+    signDate: '',
+    startDate: '',
+    endDate: '',
+    contractYears: 3,
+    probationMonths: 0,
+    probationSalary: 0,
+  })
+
+  const handleSubmit = () => {
+    const data: any = {
+      hireDate: new Date(form.hireDate).toISOString(),
+      isPregnant: form.isPregnant,
+      isInMedicalPeriod: form.isInMedicalPeriod,
+      isWorkInjured: form.isWorkInjured,
+    }
+    if (form.contractType !== 'UNSIGNED' && form.startDate) {
+      data.contract = {
+        signDate: form.signDate ? new Date(form.signDate).toISOString() : null,
+        startDate: new Date(form.startDate).toISOString(),
+        endDate: form.endDate ? new Date(form.endDate).toISOString() : null,
+        contractType: form.contractType,
+        contractYears: form.contractYears,
+        probationMonths: form.probationMonths,
+        probationSalary: form.probationSalary,
+      }
+    }
+    onSubmit(data)
+  }
+
+  return (
+    <Modal open onClose={onClose} title={`重新入职 - ${employee.name}`}>
+      <div className="space-y-3">
+        <div className="bg-blue-50 text-blue-700 text-xs px-3 py-2 rounded-md">
+          复用员工已有基本信息（姓名、部门、手机等），只需填写新入职日期和劳动合同。
+        </div>
+        <div>
+          <Label>员工</Label>
+          <div className="text-xs text-gray-600">{employee.name} - {employee.department}</div>
+        </div>
+        <div>
+          <Label>新入职日期</Label>
+          <Input
+            type="date"
+            value={form.hireDate}
+            onChange={(e) => setForm({ ...form, hireDate: e.target.value })}
+          />
+        </div>
+        <div className="flex gap-4 text-xs">
+          <label className="flex items-center gap-1">
+            <input type="checkbox" checked={form.isPregnant} onChange={(e) => setForm({ ...form, isPregnant: e.target.checked })} />
+            孕期
+          </label>
+          <label className="flex items-center gap-1">
+            <input type="checkbox" checked={form.isInMedicalPeriod} onChange={(e) => setForm({ ...form, isInMedicalPeriod: e.target.checked })} />
+            医疗期
+          </label>
+          <label className="flex items-center gap-1">
+            <input type="checkbox" checked={form.isWorkInjured} onChange={(e) => setForm({ ...form, isWorkInjured: e.target.checked })} />
+            工伤
+          </label>
+        </div>
+        <div className="border-t pt-3">
+          <Label>劳动合同</Label>
+          <Select value={form.contractType} onChange={(e) => setForm({ ...form, contractType: e.target.value as any })}>
+            <option value="FIXED">固定期限</option>
+            <option value="UNFIXED">无固定期限</option>
+            <option value="UNSIGNED">暂不签订</option>
+          </Select>
+        </div>
+        {form.contractType !== 'UNSIGNED' && (
+          <div className="space-y-2">
+            <div className="grid grid-cols-2 gap-2">
+              <div>
+                <Label>签订日期</Label>
+                <Input type="date" value={form.signDate} onChange={(e) => setForm({ ...form, signDate: e.target.value })} />
+              </div>
+              <div>
+                <Label>合同起始日</Label>
+                <Input type="date" value={form.startDate} onChange={(e) => setForm({ ...form, startDate: e.target.value })} />
+              </div>
+            </div>
+            {form.contractType === 'FIXED' && (
+              <div className="grid grid-cols-2 gap-2">
+                <div>
+                  <Label>合同到期日</Label>
+                  <Input type="date" value={form.endDate} onChange={(e) => setForm({ ...form, endDate: e.target.value })} />
+                </div>
+                <div>
+                  <Label>合同年限</Label>
+                  <Input type="number" value={form.contractYears} onChange={(e) => setForm({ ...form, contractYears: parseInt(e.target.value) || 3 })} />
+                </div>
+              </div>
+            )}
+            <div className="grid grid-cols-2 gap-2">
+              <div>
+                <Label>试用期（月）</Label>
+                <Input type="number" value={form.probationMonths} onChange={(e) => setForm({ ...form, probationMonths: parseInt(e.target.value) || 0 })} />
+              </div>
+              <div>
+                <Label>试用期工资</Label>
+                <Input type="number" value={form.probationSalary} onChange={(e) => setForm({ ...form, probationSalary: parseFloat(e.target.value) || 0 })} />
+              </div>
+            </div>
+          </div>
+        )}
+        {error && (
+          <div className="text-xs text-danger">
+            {(error as any)?.response?.data?.error?.message || '操作失败，请重试'}
+          </div>
+        )}
+        <div className="flex justify-end gap-2 pt-2">
+          <Button variant="secondary" onClick={onClose}>取消</Button>
+          <Button onClick={handleSubmit} disabled={loading}>
+            {loading ? '提交中...' : '确认入职'}
+          </Button>
+        </div>
+      </div>
+    </Modal>
+  )
+}
+
 function AddEmployeeModal({ onClose, onSubmit, loading, error }: {
   onClose: () => void
   onSubmit: (data: any) => void