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/Services/ESign/EsignAuditService.php

@@ -0,0 +1,58 @@
+<?php
+
+namespace App\Services\ESign;
+
+use App\Models\ESign\EsignAuditLog;
+use App\Services\Service;
+
+class EsignAuditService extends Service
+{
+    public function log(
+        int $contractId,
+        string $action,
+        ?int $signerId = null,
+        ?array $metadata = null
+    ): EsignAuditLog {
+        $request = request();
+
+        return EsignAuditLog::create([
+            'tenant_id' => $this->tenantId(),
+            'contract_id' => $contractId,
+            'signer_id' => $signerId,
+            'action' => $action,
+            'ip_address' => $request?->ip(),
+            'user_agent' => $request?->userAgent() ? mb_substr($request->userAgent(), 0, 500) : null,
+            'metadata' => $metadata,
+            'created_at' => now(),
+        ]);
+    }
+
+    public function logPublic(
+        int $tenantId,
+        int $contractId,
+        string $action,
+        ?int $signerId = null,
+        ?array $metadata = null
+    ): EsignAuditLog {
+        $request = request();
+
+        return EsignAuditLog::withoutGlobalScopes()->create([
+            'tenant_id' => $tenantId,
+            'contract_id' => $contractId,
+            'signer_id' => $signerId,
+            'action' => $action,
+            'ip_address' => $request?->ip(),
+            'user_agent' => $request?->userAgent() ? mb_substr($request->userAgent(), 0, 500) : null,
+            'metadata' => $metadata,
+            'created_at' => now(),
+        ]);
+    }
+
+    public function getContractLogs(int $contractId): \Illuminate\Database\Eloquent\Collection
+    {
+        return EsignAuditLog::where('contract_id', $contractId)
+            ->with('signer:id,name,email,role')
+            ->orderBy('created_at', 'desc')
+            ->get();
+    }
+}