fix: 입고 생성 시 balance_qty(잔량) 자동 계산 추가

- 발주 수량에서 기존 입고수량과 신규 입고수량을 차감하여 잔량 업데이트

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-node/src/controllers/receivingController.ts

@@ -286,6 +286,11 @@ export async function create(req: AuthenticatedRequest, res: Response) {
             received_qty = CAST(
               COALESCE(CAST(NULLIF(received_qty, '') AS numeric), 0) + $1 AS text
             ),
+            balance_qty = CAST(
+              COALESCE(CAST(NULLIF(order_qty, '') AS numeric), 0)
+              - COALESCE(CAST(NULLIF(received_qty, '') AS numeric), 0)
+              - $1 AS text
+            ),
             updated_date = NOW()
            WHERE id = $2 AND company_code = $3`,
           [item.inbound_qty || 0, item.source_id, companyCode]