feat(API): 부실채권, 재고, 입고 기능 개선

- BadDebt 컨트롤러/서비스 기능 확장
- StockService 재고 조회 로직 개선
- ProcessReceivingRequest 검증 규칙 수정
- Item, Order, CommonCode, Shipment 모델 업데이트
- TodayIssueObserverService 개선

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

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

@@ -18,7 +18,7 @@ public function rules(): array
             'receiving_date' => ['nullable', 'date'],
             'lot_no' => ['nullable', 'string', 'max:50'],
             'supplier_lot' => ['nullable', 'string', 'max:50'],
-            'receiving_location' => ['required', 'string', 'max:100'],
+            'receiving_location' => ['nullable', 'string', 'max:100'],
             'receiving_manager' => ['nullable', 'string', 'max:50'],
             'remark' => ['nullable', 'string', 'max:1000'],
         ];
@@ -30,7 +30,6 @@ public function messages(): array
             'receiving_qty.required' => __('validation.required', ['attribute' => '입고수량']),
             'receiving_qty.numeric' => __('validation.numeric', ['attribute' => '입고수량']),
             'receiving_qty.min' => __('validation.min.numeric', ['attribute' => '입고수량', 'min' => 0]),
-            'receiving_location.required' => __('validation.required', ['attribute' => '입고위치']),
         ];
     }
 }