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

login.php

@@ -31,12 +31,16 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
 
         if (empty($email) || empty($password)) {
             $error = 'Bitte E-Mail und Passwort eingeben';
-        } elseif ($auth->login($email, $password)) {
-            // Nach erfolgreichem Login zu index.php
-            header('Location: index.php');
-            exit;
         } else {
-            $error = 'Ungültige E-Mail oder Passwort';
+            $result = $auth->login($email, $password);
+            if ($result === true) {
+                header('Location: index.php');
+                exit;
+            } elseif ($result === 'rate_limited') {
+                $error = 'Zu viele Fehlversuche. Bitte warten Sie 10 Minuten.';
+            } else {
+                $error = 'Ungültige E-Mail oder Passwort';
+            }
         }
     } catch (Exception $e) {
         $error = 'Login-Fehler: ' . $e->getMessage();