Fix sites freeze bug, add features and shorten README

Bug fixes:
- UniFiController: add CURLOPT_TIMEOUT (10s) and CURLOPT_CONNECTTIMEOUT (5s)
  to login() and apiRequest() — prevents page freeze when controller unreachable
- UniFiController: fix login response validation for UniFi OS API which returns
  a user object instead of meta.rc=ok
- admin/sites.php: add JS loading state on form submit to give visual feedback
- login.php: handle new 'rate_limited' return value from Auth::login()

New features:
- Database: in-memory settings cache eliminates redundant DB queries per request
- Auth: login rate limiting (10 attempts per 10 min per IP/email) via login_attempts table
- admin/vouchers.php: CSV export with UTF-8 BOM for Excel compatibility
- admin/vouchers.php: client-side pagination (50 per page)
- index.php: QR code display after voucher creation (qrcodejs CDN)
- Mailer: sendTestEmail() method
- admin/settings.php: SMTP test button with AJAX handler
- database.sql: add login_attempts and audit_log tables

Readme: condensed from ~420 to ~220 lines, removed duplicated sections,
M365 Azure Portal walkthrough, contribution guidelines, update/migration section

https://claude.ai/code/session_01UsuvFAmmeagtQa14QA4iaq

includes/Mailer.php

@@ -201,6 +201,13 @@ class Mailer {
         return $this->send($to, $subject, $body, $isHtml);
     }
     
+    public function sendTestEmail($to) {
+        $appTitle = $this->db->getSetting('app_title', 'UniFi Voucher System');
+        $subject = '[Test] E-Mail-Konfiguration – ' . $appTitle;
+        $body = "Dies ist eine Test-E-Mail von {$appTitle}.\n\nDie SMTP-Konfiguration ist korrekt eingerichtet.";
+        return $this->send($to, $subject, $body, false);
+    }
+
     public function sendUserNotification($to, $userName, $changes) {
         $appTitle = $this->db->getSetting('app_title', 'UniFi Voucher System');