fix: 입고 등록 order_qty 필수 검증 제거

- StoreReceivingRequest에서 order_qty를 required → nullable로 변경
- 입고 등록 시 발주수량 없이도 등록 가능하도록 수정

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

app/Http/Requests/V1/Receiving/StoreReceivingRequest.php

@@ -21,7 +21,7 @@ public function rules(): array
             'item_name' => ['required', 'string', 'max:200'],
             'specification' => ['nullable', 'string', 'max:200'],
             'supplier' => ['required', 'string', 'max:100'],
-            'order_qty' => ['required', 'numeric', 'min:0'],
+            'order_qty' => ['nullable', 'numeric', 'min:0'],
             'order_unit' => ['nullable', 'string', 'max:20'],
             'due_date' => ['nullable', 'date'],
             'status' => ['nullable', 'string', 'in:order_completed,shipping,inspection_pending,receiving_pending'],