refactor(backend): update employees endpoint and service

- endpoints/employees.py: update employees API endpoint
- services/employee.py: update employee service

server/src/app/services/employee.py

@@ -333,6 +333,27 @@ class EmployeeService:
         logger.info("Disabled employee id=%s no=%s", employee.id, employee.employee_no)
         return self._serialize_employee(hydrated or saved)
 
+    def enable_employee(self, employee_id: str) -> EmployeeRead:
+        self.ensure_directory_ready()
+
+        employee = self.repository.get(employee_id)
+        if employee is None:
+            raise LookupError("Employee not found")
+
+        if employee.employment_status != "停用":
+            return self._serialize_employee(employee)
+
+        now = datetime.now()
+        employee.employment_status = "在职"
+        employee.sync_state = "已同步"
+        employee.last_sync_at = now
+        self._append_change_log(employee, action="启用员工账号", occurred_at=now)
+
+        saved = self.repository.save(employee)
+        hydrated = self.repository.get(saved.id)
+        logger.info("Enabled employee id=%s no=%s", employee.id, employee.employee_no)
+        return self._serialize_employee(hydrated or saved)
+
     def _seed_roles(self) -> None:
         existing_by_code = {role.role_code: role for role in self.repository.list_roles()}