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/Http/Requests/ESign/ContractStoreRequest.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace App\Http\Requests\ESign;
+
+use App\Models\ESign\EsignContract;
+use Illuminate\Foundation\Http\FormRequest;
+
+class ContractStoreRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'title' => 'required|string|max:200',
+            'description' => 'nullable|string|max:2000',
+            'sign_order_type' => 'nullable|string|in:' . implode(',', EsignContract::SIGN_ORDERS),
+            'file' => 'required|file|mimes:pdf|max:20480',
+            'expires_at' => 'nullable|date|after:now',
+            'creator_name' => 'required|string|max:100',
+            'creator_email' => 'required|email|max:255',
+            'creator_phone' => 'nullable|string|max:20',
+            'counterpart_name' => 'required|string|max:100',
+            'counterpart_email' => 'required|email|max:255',
+            'counterpart_phone' => 'nullable|string|max:20',
+        ];
+    }
+
+    public function messages(): array
+    {
+        return [
+            'title.required' => __('validation.required', ['attribute' => '계약 제목']),
+            'file.required' => __('validation.required', ['attribute' => 'PDF 파일']),
+            'file.mimes' => __('validation.mimes', ['attribute' => '파일', 'values' => 'PDF']),
+            'file.max' => __('validation.max.file', ['attribute' => '파일', 'max' => '20MB']),
+            'creator_name.required' => __('validation.required', ['attribute' => '작성자 이름']),
+            'creator_email.required' => __('validation.required', ['attribute' => '작성자 이메일']),
+            'counterpart_name.required' => __('validation.required', ['attribute' => '상대방 이름']),
+            'counterpart_email.required' => __('validation.required', ['attribute' => '상대방 이메일']),
+        ];
+    }
+}