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

admin/sites.php

@@ -654,6 +654,22 @@ $currentUser = $auth->getCurrentUser();
             document.getElementById('editSiteModal').classList.add('active');
         }
         
+        // Loading-State bei Formular-Absenden (verhindert Freeze-Eindruck)
+        document.getElementById('addSiteForm').addEventListener('submit', function() {
+            const btn = this.querySelector('button[name="add_site"]');
+            if (btn) {
+                btn.disabled = true;
+                btn.innerHTML = '<i class="fas fa-spinner fa-spin"></i> Verbindung wird getestet...';
+            }
+        });
+        document.getElementById('editSiteForm').addEventListener('submit', function() {
+            const btn = this.querySelector('button[name="edit_site"]');
+            if (btn) {
+                btn.disabled = true;
+                btn.innerHTML = '<i class="fas fa-spinner fa-spin"></i> Verbindung wird getestet...';
+            }
+        });
+
         // Modal schließen bei Klick außerhalb
         document.getElementById('addSiteModal').addEventListener('click', function(e) {
             if (e.target === this) {