fix: ApprovalStep 모델 Eloquent Model import 누락 수정

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/Models/Tenants/ApprovalStep.php

@@ -3,6 +3,7 @@
 namespace App\Models\Tenants;
 
 use App\Models\Members\User;
+use App\Traits\Auditable;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 
@@ -22,6 +23,7 @@
  */
 class ApprovalStep extends Model
 {
+    use Auditable;
     protected $table = 'approval_steps';
 
     protected $casts = [