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/UniFiController.php

@@ -36,6 +36,8 @@ class UniFiController {
             CURLOPT_SSL_VERIFYPEER => false,
             CURLOPT_COOKIEJAR => $this->cookieFile,
             CURLOPT_COOKIEFILE => $this->cookieFile,
+            CURLOPT_TIMEOUT => 10,
+            CURLOPT_CONNECTTIMEOUT => 5,
             CURLOPT_HTTPHEADER => ['Content-Type: application/json'],
             CURLOPT_HEADERFUNCTION => function($ch, $header) {
                 $parts = explode(':', $header, 2);
@@ -59,11 +61,15 @@ class UniFiController {
         }
         
         $data = json_decode($response, true);
-        
-        if (!isset($data['meta']['rc']) || $data['meta']['rc'] !== 'ok') {
-            throw new Exception("Login fehlgeschlagen: Ungültige Antwort");
+
+        // UniFi OS gibt ein User-Objekt zurück (unique_id/email), die alte API meta.rc = ok
+        $isUnifiOs = is_array($data) && (isset($data['unique_id']) || isset($data['email']));
+        $isOldApi  = isset($data['meta']['rc']) && $data['meta']['rc'] === 'ok';
+
+        if (!$isUnifiOs && !$isOldApi) {
+            throw new Exception("Login fehlgeschlagen: Ungültige Antwort vom Controller");
         }
-        
+
         return true;
     }
     
@@ -79,6 +85,8 @@ class UniFiController {
             CURLOPT_RETURNTRANSFER => true,
             CURLOPT_SSL_VERIFYPEER => false,
             CURLOPT_COOKIEFILE => $this->cookieFile,
+            CURLOPT_TIMEOUT => 10,
+            CURLOPT_CONNECTTIMEOUT => 5,
             CURLOPT_HTTPHEADER => array_filter([
                 'Content-Type: application/json',
                 ($method === 'POST' && $this->csrfToken !== null)