feat:E-Sign 전자계약 서명 솔루션 백엔드 구현

- 마이그레이션 4개 (esign_contracts, esign_signers, esign_sign_fields, esign_audit_logs)
- 모델 4개 (EsignContract, EsignSigner, EsignSignField, EsignAuditLog)
- 서비스 4개 (EsignContractService, EsignSignService, EsignPdfService, EsignAuditService)
- 컨트롤러 2개 (EsignContractController, EsignSignController)
- FormRequest 4개 (ContractStore, FieldConfigure, SignSubmit, SignReject)
- Mail 1개 (EsignRequestMail + 이메일 템플릿)
- API 라우트 (인증 계약 관리 + 토큰 기반 서명 프로세스)

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

app/Models/ESign/EsignSignField.php

@@ -0,0 +1,67 @@
+<?php
+
+namespace App\Models\ESign;
+
+use App\Traits\BelongsToTenant;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class EsignSignField extends Model
+{
+    use BelongsToTenant;
+
+    protected $table = 'esign_sign_fields';
+
+    // 필드 유형 상수
+    public const TYPE_SIGNATURE = 'signature';
+    public const TYPE_STAMP = 'stamp';
+    public const TYPE_TEXT = 'text';
+    public const TYPE_DATE = 'date';
+    public const TYPE_CHECKBOX = 'checkbox';
+
+    public const FIELD_TYPES = [
+        self::TYPE_SIGNATURE,
+        self::TYPE_STAMP,
+        self::TYPE_TEXT,
+        self::TYPE_DATE,
+        self::TYPE_CHECKBOX,
+    ];
+
+    protected $fillable = [
+        'tenant_id',
+        'contract_id',
+        'signer_id',
+        'page_number',
+        'position_x',
+        'position_y',
+        'width',
+        'height',
+        'field_type',
+        'field_label',
+        'field_value',
+        'is_required',
+        'sort_order',
+    ];
+
+    protected $casts = [
+        'page_number' => 'integer',
+        'position_x' => 'decimal:2',
+        'position_y' => 'decimal:2',
+        'width' => 'decimal:2',
+        'height' => 'decimal:2',
+        'is_required' => 'boolean',
+        'sort_order' => 'integer',
+    ];
+
+    // === Relations ===
+
+    public function contract(): BelongsTo
+    {
+        return $this->belongsTo(EsignContract::class, 'contract_id');
+    }
+
+    public function signer(): BelongsTo
+    {
+        return $this->belongsTo(EsignSigner::class, 'signer_id');
+    }
+}