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>

app/Models/ESign/EsignContract.php

@@ -57,6 +57,7 @@ class EsignContract extends Model
         'signed_file_path',
         'signed_file_hash',
         'status',
+        'metadata',
         'expires_at',
         'completed_at',
         'created_by',
@@ -66,6 +67,7 @@ class EsignContract extends Model
 
     protected $casts = [
         'original_file_size' => 'integer',
+        'metadata' => 'array',
         'expires_at' => 'datetime',
         'completed_at' => 'datetime',
     ];

app/Models/ESign/EsignSignField.php

@@ -38,6 +38,7 @@ class EsignSignField extends Model
         'height',
         'field_type',
         'field_label',
+        'field_variable',
         'font_size',
         'field_value',
         'is_required',