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/handlers/received-invoices-handlers.php

@@ -358,7 +358,12 @@ function handleBulkUpload(PDO $pdo, array $authData): void
 
 function handleUpdateReceivedInvoice(PDO $pdo, int $id): void
 {
-    $stmt = $pdo->prepare('SELECT * FROM received_invoices WHERE id = ?');
+    $stmt = $pdo->prepare(
+        'SELECT id, supplier_name, invoice_number, description,
+                amount, currency, vat_rate, vat_amount,
+                issue_date, due_date, paid_date, status, notes
+        FROM received_invoices WHERE id = ?'
+    );
     $stmt->execute([$id]);
     $invoice = $stmt->fetch();