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/Mail/EsignRequestMail.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace App\Mail;
+
+use App\Models\ESign\EsignContract;
+use App\Models\ESign\EsignSigner;
+use Illuminate\Bus\Queueable;
+use Illuminate\Mail\Mailable;
+use Illuminate\Mail\Mailables\Content;
+use Illuminate\Mail\Mailables\Envelope;
+use Illuminate\Queue\SerializesModels;
+
+class EsignRequestMail extends Mailable
+{
+    use Queueable, SerializesModels;
+
+    public function __construct(
+        public EsignContract $contract,
+        public EsignSigner $signer,
+    ) {}
+
+    public function envelope(): Envelope
+    {
+        return new Envelope(
+            subject: "[SAM] 전자계약 서명 요청 - {$this->contract->title}",
+        );
+    }
+
+    public function content(): Content
+    {
+        $signUrl = config('app.mng_url', config('app.url'))
+            . '/esign/sign/' . $this->signer->access_token;
+
+        return new Content(
+            html: 'emails.esign.request',
+            with: [
+                'contractTitle' => $this->contract->title,
+                'signerName' => $this->signer->name,
+                'signUrl' => $signUrl,
+                'expiresAt' => $this->contract->expires_at?->format('Y-m-d H:i'),
+            ],
+        );
+    }
+}