feat:서명 완료 시 계약서 이메일 발송 및 감사 로그 추가

- EsignCompletedMail Mailable 생성 (완료 알림 + PDF 다운로드 링크)
- completed.blade.php 이메일 뷰 템플릿 생성 (초록색 테마)
- submitSignature에 contract_completed 감사 로그 추가
- 모든 서명자에게 완료 이메일 발송 + completion_email_sent 감사 로그
- 이메일 발송 실패 시 try-catch로 계약 완료 보호

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

app/Mail/EsignCompletedMail.php

@@ -0,0 +1,46 @@
+<?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;
+use Illuminate\Support\Collection;
+
+class EsignCompletedMail extends Mailable
+{
+    use Queueable, SerializesModels;
+
+    public function __construct(
+        public EsignContract $contract,
+        public EsignSigner $signer,
+        public Collection $allSigners,
+    ) {}
+
+    public function envelope(): Envelope
+    {
+        return new Envelope(
+            subject: "[SAM] 전자계약 서명 완료 - {$this->contract->title}",
+        );
+    }
+
+    public function content(): Content
+    {
+        $downloadUrl = config('app.url') . '/esign/sign/' . $this->signer->access_token . '/api/document';
+
+        return new Content(
+            html: 'emails.esign.completed',
+            with: [
+                'contractTitle' => $this->contract->title,
+                'signerName' => $this->signer->name,
+                'completedAt' => $this->contract->completed_at?->format('Y-m-d H:i'),
+                'allSigners' => $this->allSigners,
+                'downloadUrl' => $downloadUrl,
+            ],
+        );
+    }
+}