Add comprehensive unit tests for invoicing period cache logic, system search behavior, and OpenAPI spec coverage. Expand search service with generic entity support and refine association type handling.

services/nginx/app/objects/users_o.php

@@ -1208,7 +1208,7 @@ class users_o extends db
         }
 
         $customerNumbers = [];
-        $sql = "SELECT customer_number FROM `$this->table` WHERE id IN (" . implode(',', array_map('intval', $userIds)) . ")";
+        $sql = "SELECT customer_number FROM $this->table WHERE id IN (" . implode(',', array_map('intval', $userIds)) . ")";
         $result = $db->query($sql);
         while ($row = $result->fetch_assoc()) {
             $customerNumbers[] = (int)$row['customer_number'];