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/EsignAuditLog.php

@@ -0,0 +1,57 @@
+<?php
+
+namespace App\Models\ESign;
+
+use App\Traits\BelongsToTenant;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class EsignAuditLog extends Model
+{
+    use BelongsToTenant;
+
+    protected $table = 'esign_audit_logs';
+
+    public $timestamps = false;
+
+    // 액션 상수
+    public const ACTION_CREATED = 'created';
+    public const ACTION_SENT = 'sent';
+    public const ACTION_VIEWED = 'viewed';
+    public const ACTION_OTP_SENT = 'otp_sent';
+    public const ACTION_AUTHENTICATED = 'authenticated';
+    public const ACTION_SIGNED = 'signed';
+    public const ACTION_REJECTED = 'rejected';
+    public const ACTION_COMPLETED = 'completed';
+    public const ACTION_CANCELLED = 'cancelled';
+    public const ACTION_REMINDED = 'reminded';
+    public const ACTION_DOWNLOADED = 'downloaded';
+
+    protected $fillable = [
+        'tenant_id',
+        'contract_id',
+        'signer_id',
+        'action',
+        'ip_address',
+        'user_agent',
+        'metadata',
+        'created_at',
+    ];
+
+    protected $casts = [
+        'metadata' => 'array',
+        'created_at' => 'datetime',
+    ];
+
+    // === Relations ===
+
+    public function contract(): BelongsTo
+    {
+        return $this->belongsTo(EsignContract::class, 'contract_id');
+    }
+
+    public function signer(): BelongsTo
+    {
+        return $this->belongsTo(EsignSigner::class, 'signer_id');
+    }
+}