feat:E-Sign 템플릿 변수 시스템 추가 (마이그레이션+모델)

- field_variable, metadata, variables 컬럼 마이그레이션 추가
- EsignContract 모델에 metadata (JSON cast) 추가
- EsignSignField 모델에 field_variable 추가

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

database/migrations/2026_02_13_080000_add_variable_system_to_esign_tables.php

@@ -0,0 +1,50 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        // 템플릿에 변수 정의 JSON 추가
+        Schema::table('esign_field_templates', function (Blueprint $table) {
+            $table->json('variables')->nullable()->after('signer_count');
+        });
+
+        // 템플릿 필드에 변수 참조 추가
+        Schema::table('esign_field_template_items', function (Blueprint $table) {
+            $table->string('field_variable', 50)->nullable()->after('field_label');
+        });
+
+        // 계약에 메타데이터 JSON 추가
+        Schema::table('esign_contracts', function (Blueprint $table) {
+            $table->json('metadata')->nullable()->after('status');
+        });
+
+        // 계약 필드에 변수 참조 추가
+        Schema::table('esign_sign_fields', function (Blueprint $table) {
+            $table->string('field_variable', 50)->nullable()->after('field_label');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('esign_sign_fields', function (Blueprint $table) {
+            $table->dropColumn('field_variable');
+        });
+
+        Schema::table('esign_contracts', function (Blueprint $table) {
+            $table->dropColumn('metadata');
+        });
+
+        Schema::table('esign_field_template_items', function (Blueprint $table) {
+            $table->dropColumn('field_variable');
+        });
+
+        Schema::table('esign_field_templates', function (Blueprint $table) {
+            $table->dropColumn('variables');
+        });
+    }
+};