fix:테넌트 전환 시 고유 코드 자동 생성

- code 필드 추가 (T + 날짜 + 랜덤 4자리)
- 예: T260128ABC1

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/Services/Sales/TenantProspectService.php

@@ -103,9 +103,13 @@ public function update(
     public function convertToTenant(TenantProspect $prospect, int $convertedBy): Tenant
     {
         return DB::transaction(function () use ($prospect, $convertedBy) {
+            // 고유 테넌트 코드 생성 (T + 타임스탬프 + 랜덤)
+            $tenantCode = 'T' . now()->format('ymd') . strtoupper(substr(uniqid(), -4));
+
             // 테넌트 생성
             $tenant = Tenant::create([
                 'company_name' => $prospect->company_name,
+                'code' => $tenantCode,
                 'business_num' => $prospect->business_number,
                 'ceo_name' => $prospect->ceo_name,
                 'phone' => $prospect->contact_phone,