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/Auth.php

@@ -23,19 +23,62 @@ class Auth {
     
     // Benutzer einloggen
     public function login($email, $password) {
+        $ip = $_SERVER['REMOTE_ADDR'] ?? '0.0.0.0';
+
+        if ($this->isRateLimited($ip, $email)) {
+            return 'rate_limited';
+        }
+
         $user = $this->db->fetchOne(
             "SELECT * FROM users WHERE email = ? AND is_active = 1",
             [$email]
         );
-        
+
         if ($user && password_verify($password, $user['password_hash'])) {
+            $this->clearLoginAttempts($ip, $email);
             $this->setUserSession($user);
             $this->updateLastLogin($user['id']);
             return true;
         }
-        
+
+        $this->recordLoginAttempt($ip, $email);
         return false;
     }
+
+    private function isRateLimited($ip, $email) {
+        try {
+            $count = $this->db->fetchOne(
+                "SELECT COUNT(*) as cnt FROM login_attempts
+                 WHERE (ip_address = ? OR email = ?) AND attempted_at > DATE_SUB(NOW(), INTERVAL 10 MINUTE)",
+                [$ip, $email]
+            );
+            return $count && (int)$count['cnt'] >= 10;
+        } catch (\Exception $e) {
+            return false;
+        }
+    }
+
+    private function recordLoginAttempt($ip, $email) {
+        try {
+            $this->db->query(
+                "INSERT INTO login_attempts (ip_address, email) VALUES (?, ?)",
+                [$ip, $email]
+            );
+        } catch (\Exception $e) {
+            // Tabelle existiert noch nicht – ignorieren
+        }
+    }
+
+    private function clearLoginAttempts($ip, $email) {
+        try {
+            $this->db->query(
+                "DELETE FROM login_attempts WHERE ip_address = ? OR email = ?",
+                [$ip, $email]
+            );
+        } catch (\Exception $e) {
+            // ignore
+        }
+    }
     
     // Microsoft 365 Login
     public function loginWithMicrosoft($microsoftUser) {