feat: Registrierung default AN, Einladungslinks, System-Email

- Registrierung ist standardmaessig aktiviert (erster User = Admin)
- Einmal-Registrierungslinks: Admin kann Links generieren die auch bei
  deaktivierter Registrierung funktionieren, nach Nutzung ungueltig
- Optional Link per System-Email versenden
- System-SMTP in Admin-Einstellungen konfigurierbar:
  Server, Port, SSL, Benutzername, Passwort, Absender-Adresse
- SMTP-Verbindungstest-Button
- Register-Seite akzeptiert ?invite=TOKEN aus der URL

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/users.py

@@ -138,7 +138,13 @@ def delete_user(user_id):
 @admin_required
 def get_settings():
     return jsonify({
-        'public_registration': AppSettings.get_bool('public_registration', default=False),
+        'public_registration': AppSettings.get_bool('public_registration', default=True),
+        'system_smtp_host': AppSettings.get('system_smtp_host', ''),
+        'system_smtp_port': int(AppSettings.get('system_smtp_port', '587')),
+        'system_smtp_ssl': AppSettings.get_bool('system_smtp_ssl', default=True),
+        'system_smtp_username': AppSettings.get('system_smtp_username', ''),
+        'system_smtp_password_set': bool(AppSettings.get('system_smtp_password', '')),
+        'system_email_from': AppSettings.get('system_email_from', ''),
     }), 200
 
 
@@ -148,9 +154,110 @@ def update_settings():
     data = request.get_json()
     if 'public_registration' in data:
         AppSettings.set('public_registration', str(data['public_registration']).lower())
+    for key in ['system_smtp_host', 'system_smtp_port', 'system_smtp_ssl',
+                'system_smtp_username', 'system_email_from']:
+        if key in data:
+            AppSettings.set(key, str(data[key]))
+    if 'system_smtp_password' in data and data['system_smtp_password']:
+        AppSettings.set('system_smtp_password', data['system_smtp_password'])
     return jsonify({'message': 'Einstellungen gespeichert'}), 200
 
 
+@api_bp.route('/settings/test-email', methods=['POST'])
+@admin_required
+def test_system_email():
+    """Test system SMTP connection."""
+    import smtplib
+
+    host = AppSettings.get('system_smtp_host', '')
+    port = int(AppSettings.get('system_smtp_port', '587'))
+    ssl = AppSettings.get_bool('system_smtp_ssl', default=True)
+    username = AppSettings.get('system_smtp_username', '')
+    password = AppSettings.get('system_smtp_password', '')
+    from_addr = AppSettings.get('system_email_from', '')
+
+    if not host or not username or not password:
+        return jsonify({'error': 'SMTP-Einstellungen unvollstaendig'}), 400
+
+    try:
+        if ssl and port == 465:
+            server = smtplib.SMTP_SSL(host, port, timeout=10)
+        else:
+            server = smtplib.SMTP(host, port, timeout=10)
+            server.starttls()
+        server.login(username, password)
+        server.quit()
+        return jsonify({'message': 'SMTP-Verbindung erfolgreich'}), 200
+    except Exception as e:
+        return jsonify({'error': f'Verbindungsfehler: {str(e)}'}), 400
+
+
+# --- Invite Links ---
+
+@api_bp.route('/settings/invite', methods=['POST'])
+@admin_required
+def create_invite_link():
+    """Generate a one-time registration link."""
+    import secrets
+    token = secrets.token_urlsafe(32)
+    AppSettings.set(f'invite_{token}', 'valid')
+
+    data = request.get_json() or {}
+    send_email = data.get('send_to_email', '')
+
+    result = {
+        'token': token,
+        'url': f'/register?invite={token}',
+    }
+
+    # Optionally send via system email
+    if send_email:
+        import smtplib
+        from email.mime.text import MIMEText
+
+        host = AppSettings.get('system_smtp_host', '')
+        port = int(AppSettings.get('system_smtp_port', '587'))
+        ssl = AppSettings.get_bool('system_smtp_ssl', default=True)
+        username = AppSettings.get('system_smtp_username', '')
+        password = AppSettings.get('system_smtp_password', '')
+        from_addr = AppSettings.get('system_email_from', '')
+
+        if not host or not password:
+            return jsonify({**result, 'email_sent': False,
+                          'email_error': 'System-Email nicht konfiguriert'}), 200
+
+        # Build full URL from request
+        base_url = request.host_url.rstrip('/')
+        full_url = f'{base_url}/register?invite={token}'
+
+        body = (
+            f'Du wurdest zur Mini-Cloud eingeladen!\n\n'
+            f'Klicke auf folgenden Link, um dich zu registrieren:\n'
+            f'{full_url}\n\n'
+            f'Dieser Link ist nur einmal verwendbar.'
+        )
+        msg = MIMEText(body, 'plain', 'utf-8')
+        msg['From'] = from_addr or username
+        msg['To'] = send_email
+        msg['Subject'] = 'Einladung zur Mini-Cloud'
+
+        try:
+            if ssl and port == 465:
+                server = smtplib.SMTP_SSL(host, port, timeout=10)
+            else:
+                server = smtplib.SMTP(host, port, timeout=10)
+                server.starttls()
+            server.login(username, password)
+            server.sendmail(from_addr or username, [send_email], msg.as_string())
+            server.quit()
+            result['email_sent'] = True
+        except Exception as e:
+            result['email_sent'] = False
+            result['email_error'] = str(e)
+
+    return jsonify(result), 201
+
+
 # --- User search (for sharing dialogs) ---
 
 @api_bp.route('/users/search', methods=['GET'])