feat: 员工同步增加离职处理功能

- 全量同步 (sync_employees) 现在会检测并软删除离职员工
- 增量同步改为软删除而非物理删除，更安全
- 离职处理：设置 is_active=False, is_deleted=True
- 前端显示离职处理数量统计

frontend/src/views/admin/system-settings.vue

@@ -348,12 +348,14 @@ const triggerSync = async () => {
       const created = data.users_created || 0
       const existing = data.users_existing || 0
       const restored = data.users_restored || 0
+      const departed = data.users_departed || 0
       const skipped = data.users_skipped || 0
       
-      let msg = `同步完成！共处理 ${data.total_employees || 0} 名员工`
+      let msg = `同步完成！钉钉在职 ${data.total_employees || 0} 人`
       if (created > 0) msg += `，新增 ${created} 人`
       if (existing > 0) msg += `，已存在 ${existing} 人`
       if (restored > 0) msg += `，恢复 ${restored} 人`
+      if (departed > 0) msg += `，离职 ${departed} 人`
       if (skipped > 0) msg += `，跳过 ${skipped} 人`
       
       ElMessage.success(msg)