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

database/migrations/2026_03_11_100001_add_tenant_id_and_soft_deletes_to_approval_steps_table.php

@@ -0,0 +1,34 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('approval_steps', function (Blueprint $table) {
+            $table->unsignedBigInteger('tenant_id')->nullable()->after('id')->comment('테넌트 ID');
+            $table->softDeletes()->comment('삭제일시');
+
+            $table->index('tenant_id', 'idx_approval_steps_tenant');
+        });
+
+        // 기존 데이터: 부모 approvals 테이블에서 tenant_id 복사
+        DB::statement('
+            UPDATE approval_steps AS s
+            INNER JOIN approvals AS a ON s.approval_id = a.id
+            SET s.tenant_id = a.tenant_id
+        ');
+    }
+
+    public function down(): void
+    {
+        Schema::table('approval_steps', function (Blueprint $table) {
+            $table->dropIndex('idx_approval_steps_tenant');
+            $table->dropColumn(['tenant_id', 'deleted_at']);
+        });
+    }
+};