feat: P4 backend kvalita - SELECT * fix, overdue konsolidace, Validator

- SELECT * nahrazen explicitnimi sloupci ve 22 PHP souborech (69+ vyskytu)
- users-handlers.php: password_hash explicitne vyloucen z dotazu
- Overdue detekce presunuta do invoices.php routeru (1x pred dispatch misto 3x v handlerech)
- Validator.php: validacni helper s pravidly required, string, int, email, in, numeric
- PaginationHelper: PHPStan typy opraveny

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api/admin/invoices.php

@@ -37,6 +37,11 @@ $id = isset($_GET['id']) ? (int) $_GET['id'] : null;
 try {
     $pdo = db();
 
+    // Overdue detekce - jednou pred dispatchem (misto v kazdem handleru)
+    if ($method === 'GET') {
+        $pdo->exec("UPDATE invoices SET status = 'overdue' WHERE status = 'issued' AND due_date < CURDATE()");
+    }
+
     switch ($method) {
         case 'GET':
             requirePermission($authData, 'invoices.view');