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/Database.php

@@ -2,6 +2,7 @@
 class Database {
     private static $instance = null;
     private $pdo;
+    private $settingsCache = [];
     
     private function __construct() {
         try {
@@ -58,15 +59,21 @@ class Database {
     
     // Settings-Helper
     public function getSetting($key, $default = null) {
+        if (array_key_exists($key, $this->settingsCache)) {
+            return $this->settingsCache[$key] ?? $default;
+        }
         $result = $this->fetchOne("SELECT setting_value FROM settings WHERE setting_key = ?", [$key]);
-        return $result ? $result['setting_value'] : $default;
+        $value = $result ? $result['setting_value'] : null;
+        $this->settingsCache[$key] = $value;
+        return $value ?? $default;
     }
-    
+
     public function setSetting($key, $value) {
         $this->query(
-            "INSERT INTO settings (setting_key, setting_value) VALUES (?, ?) 
+            "INSERT INTO settings (setting_key, setting_value) VALUES (?, ?)
              ON DUPLICATE KEY UPDATE setting_value = VALUES(setting_value)",
             [$key, $value]
         );
+        $this->settingsCache[$key] = $value;
     }
 }