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/users-handlers.php

@@ -142,7 +142,11 @@ function handleCreateUser(PDO $pdo, array $authData): void
 function handleUpdateUser(PDO $pdo, int $userId, int $currentUserId, array $authData): void
 {
     // Get existing user
-    $stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?');
+    $stmt = $pdo->prepare('
+        SELECT id, username, email, first_name, last_name, role_id, is_active,
+               last_login, created_at
+        FROM users WHERE id = ?
+    ');
     $stmt->execute([$userId]);
     $existingUser = $stmt->fetch();