refactor: [approval] SAM API 규칙 준수 코드 리뷰 반영

- ApprovalStep에 BelongsToTenant, SoftDeletes 추가 (마이그레이션 포함)
- ApprovalForm, ApprovalDelegation에 ModelTrait 추가 (중복 scopeActive 제거)
- ApprovalDelegation에 Auditable 추가
- 모든 결재 액션에 FormRequest 적용 (approve, cancel, hold, preDecide)
- 위임 CRUD에 DelegationStoreRequest, DelegationUpdateRequest 적용
- ApprovalStep 생성 시 tenant_id 포함

app/Http/Requests/Approval/DelegationStoreRequest.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Http\Requests\Approval;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class DelegationStoreRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'delegate_id' => 'required|integer|exists:users,id',
+            'start_date' => 'required|date|after_or_equal:today',
+            'end_date' => 'required|date|after_or_equal:start_date',
+            'form_ids' => 'nullable|array',
+            'form_ids.*' => 'integer|exists:approval_forms,id',
+            'notify_delegator' => 'nullable|boolean',
+            'reason' => 'nullable|string|max:500',
+        ];
+    }
+}