refactor: 删除 users 表中未使用的字段

- 删除 report_to（上级ID）字段
- 删除 position（职位）字段
- 更新 User 模型移除相关关联和类型转换
- 更新 BusinessTripService 移除 position 返回

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/model/User.php

@@ -14,7 +14,6 @@ class User extends Model
     protected $fillable = [
         'company_id',
         'sn',
-        'report_to',
         'name',
         'avatar',
         'sex',
@@ -25,13 +24,11 @@ class User extends Model
         'user_type',
         'union_id',         // 飞书 UnionID - 用于跨应用识别用户
         'feishu_app_id',    // 最后登录的飞书应用ID - 用于快速获取默认应用
-        'position',
         'sign_img',
     ];
 
     protected $casts = [
         'company_id' => 'integer',
-        'report_to' => 'integer',
         'sex' => 'integer',
         'status' => 'integer',
         'user_type' => 'integer',
@@ -51,22 +48,6 @@ class User extends Model
         return $this->belongsTo(Company::class, 'company_id');
     }
 
-    /**
-     * 上级用户
-     */
-    public function parent()
-    {
-        return $this->belongsTo(User::class, 'report_to');
-    }
-
-    /**
-     * 下级用户
-     */
-    public function children()
-    {
-        return $this->hasMany(User::class, 'report_to');
-    }
-
     /**
      * 用户飞书应用绑定
      */