新增 POS 庫存查詢 API：實作 InventorySyncController 與相關 Service 邏輯，並更新 API 整合手冊

app/Modules/Inventory/Contracts/InventoryServiceInterface.php

@@ -157,4 +157,12 @@ interface InventoryServiceInterface
      * Get items expiring soon for dashboard.
      */
     public function getExpiringSoon(int $limit = 5): \Illuminate\Support\Collection;
+
+    /**
+     * Get inventory summary (group by product) for a specific warehouse code
+     * 
+     * @param string $code
+     * @return \Illuminate\Support\Collection|null
+     */
+    public function getPosInventoryByWarehouseCode(string $code);
 }