fix: add persistent connections + fix self::$pdo bug

- Add PDO::ATTR_PERSISTENT to prevent SQL Server connection storms
  under concurrent PHP-FPM load (was causing 241 SQLSTATE[08001]
  failures/day on callioapi)
- Fix bug: $pdo → self::$pdo in connect() and getPDOInstance()
  guards (was checking undefined local var instead of static prop)

src/PDOWithFilters/PDOWithFilters.php

@@ -35,16 +35,16 @@ class PDOWithFilters {
 
 
   public static function connect($dsn, $username, $password) {
-    if (!empty($pdo)) {
+    if (!empty(self::$pdo)) {
       throw new \Exception('Create Pdo only once');
     }
-    self::$pdo = new \PDO($dsn, $username, $password);
+    self::$pdo = new \PDO($dsn, $username, $password, [\PDO::ATTR_PERSISTENT => true]);
 
     return self::$pdo;
   }
 
   public static function getPDOInstance() {
-    if (empty($pdo)) {
+    if (empty(self::$pdo)) {
       throw new \Exception('You must call connect() first');
     }
     return self::$pdo;