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/profile.php

@@ -35,7 +35,11 @@ try {
     $userId = $authData['user_id'];
 
     // 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();