more OOP and restructure utils

app/inventory/api/engine/product.php

@@ -3,10 +3,14 @@
 
     require '../../../assets/utils/db_auth.php';
 
-    $sql = "SELECT * 
-        FROM md_product
+    $sql = "SELECT a.* , IFNULL(SUM(b.total_in-b.total_out),0) AS product_balance
+        FROM md_product a
+        LEFT JOIN warehouse_balance b ON 
+            a.company_id = b.company_id AND
+            a.sku = b.product_sku
         WHERE 
-            company_id = {$company_id}";
+            a.company_id = {$company_id}
+        GROUP BY a.id";
     $sth = $pdo2->prepare($sql);
     $sth->execute([
         ":company_id" => $company_id