feat: [문서] document_data, document_approvals, document_attachments에 tenant_id 추가

- 3개 테이블에 tenant_id 컬럼 + 인덱스 추가
- 기존 데이터는 부모 테이블(documents)에서 tenant_id 자동 채움
- 멀티테넌시 일관성 확보 및 데이터 동기화 지원

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

database/migrations/2026_03_10_224806_add_tenant_id_to_document_detail_tables.php

@@ -0,0 +1,46 @@
+<?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
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        $tables = ['document_data', 'document_approvals', 'document_attachments'];
+
+        foreach ($tables as $table) {
+            Schema::table($table, function (Blueprint $table) {
+                $table->unsignedBigInteger('tenant_id')->nullable()->after('id')->comment('테넌트 ID');
+                $table->index('tenant_id');
+            });
+
+            // 부모 테이블(documents)에서 tenant_id 채우기
+            DB::statement("
+                UPDATE {$table} t
+                JOIN documents d ON t.document_id = d.id
+                SET t.tenant_id = d.tenant_id
+            ");
+        }
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        $tables = ['document_data', 'document_approvals', 'document_attachments'];
+
+        foreach ($tables as $table) {
+            Schema::table($table, function (Blueprint $table) {
+                $table->dropIndex(['tenant_id']);
+                $table->dropColumn('tenant_id');
+            });
+        }
+    }
+};