feat:esign_contracts 테이블에 send_method, sms_fallback 컬럼 추가

- send_method: 발송 방식 (alimtalk/email/both), 기본값 alimtalk
- sms_fallback: SMS 대체발송 여부, 기본값 true

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

database/migrations/2026_02_14_150000_add_send_method_to_esign_contracts_table.php

@@ -0,0 +1,25 @@
+<?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('send_method', 20)->default('alimtalk')->after('status')
+                ->comment('발송 방식: alimtalk, email, both');
+            $table->boolean('sms_fallback')->default(true)->after('send_method')
+                ->comment('SMS 대체발송 사용 여부');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('esign_contracts', function (Blueprint $table) {
+            $table->dropColumn(['send_method', 'sms_fallback']);
+        });
+    }
+};