Fix PDO type hint in ItemController to use global PDO class

src/Controllers/ItemController.php

@@ -143,7 +143,7 @@ class ItemController
         }
     }
 
-    private static function generateUniqueIdCode(PDO $db): string {
+    private static function generateUniqueIdCode(\PDO $db): string {
         do {
             $code = strtoupper(substr(md5(uniqid(mt_rand(), true)), 0, 8));
             $stmt = $db->prepare('SELECT id FROM items WHERE id_code = :code');