feat: [esign] esign_contracts 테이블에 completion_template_name 컬럼 추가

- 완료 알림톡 템플릿명을 저장하기 위한 nullable string 컬럼

database/migrations/2026_02_27_120000_add_completion_template_to_esign_contracts.php

@@ -0,0 +1,23 @@
+<?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
+    {
+        Schema::table('esign_contracts', function (Blueprint $table) {
+            $table->string('completion_template_name', 100)->nullable()->after('sms_fallback')
+                ->comment('완료 알림톡 템플릿명');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('esign_contracts', function (Blueprint $table) {
+            $table->dropColumn('completion_template_name');
+        });
+    }
+};