feat:E-Sign 필드 템플릿 PDF 파일 컬럼 추가

- esign_field_templates 테이블에 file_path, file_name, file_hash, file_size 컬럼 추가
- 템플릿에 PDF 파일을 포함할 수 있도록 스키마 확장

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

database/migrations/2026_02_12_220000_add_file_to_esign_field_templates.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_field_templates', function (Blueprint $table) {
+            $table->string('file_path')->nullable()->after('category')->comment('PDF 파일 경로');
+            $table->string('file_name')->nullable()->after('file_path')->comment('원본 파일명');
+            $table->string('file_hash', 64)->nullable()->after('file_name')->comment('SHA256');
+            $table->unsignedBigInteger('file_size')->nullable()->after('file_hash')->comment('바이트');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('esign_field_templates', function (Blueprint $table) {
+            $table->dropColumn(['file_path', 'file_name', 'file_hash', 'file_size']);
+        });
+    }
+};