feat: Implement TOTP-based Two-Factor Authentication

- Add django-otp and qrcode dependencies
- Create comprehensive 2FA views and templates in German
- Add 2FA setup, verification, and management interfaces
- Implement backup token system with 10 recovery codes
- Add TwoFactorMiddleware for session enforcement
- Integrate 2FA controls into user navigation menu
- Support QR code generation for authenticator apps
- Add forms for secure 2FA operations with validation
- Configure OTP settings and admin site integration

Features:
- Optional 2FA (users can enable/disable)
- TOTP compatible with Google Authenticator, Authy, etc.
- Backup codes for emergency access
- German language interface
- Session-based 2FA enforcement
- Password confirmation for sensitive operations
- Production-ready with HTTPS support

app/templates/stiftung/auth/two_factor_verify.html

@@ -0,0 +1,89 @@
+{% extends "base.html" %}
+{% load static %}
+
+{% block title %}{{ title }}{% endblock %}
+
+{% block content %}
+<div class="container-fluid">
+    <div class="row justify-content-center">
+        <div class="col-md-6 col-lg-4">
+            <div class="card shadow">
+                <div class="card-header text-center">
+                    <h4 class="mb-0">
+                        <i class="fas fa-shield-alt text-primary"></i>
+                        Zwei-Faktor-Authentifizierung
+                    </h4>
+                </div>
+                <div class="card-body">
+                    <p class="text-center text-muted mb-4">
+                        Geben Sie den 6-stelligen Code aus Ihrer Authenticator-App ein
+                        oder verwenden Sie einen Backup-Code.
+                    </p>
+                    
+                    <form method="post">
+                        {% csrf_token %}
+                        {% if next %}
+                            <input type="hidden" name="next" value="{{ next }}">
+                        {% endif %}
+                        
+                        <div class="mb-4">
+                            <label for="otp_token" class="form-label">Authentifizierungscode</label>
+                            <input type="text" 
+                                   class="form-control form-control-lg text-center" 
+                                   id="otp_token" 
+                                   name="otp_token" 
+                                   placeholder="000000"
+                                   maxlength="8" 
+                                   required
+                                   autocomplete="off"
+                                   autofocus>
+                            <div class="form-text text-center">
+                                6-stelliger Code aus der App oder 8-stelliger Backup-Code
+                            </div>
+                        </div>
+                        
+                        <div class="d-grid">
+                            <button type="submit" class="btn btn-primary btn-lg">
+                                <i class="fas fa-sign-in-alt"></i>
+                                Bestätigen
+                            </button>
+                        </div>
+                    </form>
+                    
+                    <div class="text-center mt-4">
+                        <details>
+                            <summary class="text-muted small">Probleme beim Anmelden?</summary>
+                            <div class="mt-2 small text-muted">
+                                <p>Falls Sie keinen Zugriff auf Ihre Authenticator-App haben:</p>
+                                <ul class="text-start">
+                                    <li>Verwenden Sie einen der 8-stelligen Backup-Codes</li>
+                                    <li>Kontaktieren Sie den Administrator</li>
+                                </ul>
+                            </div>
+                        </details>
+                    </div>
+                </div>
+            </div>
+        </div>
+    </div>
+</div>
+
+<script>
+document.addEventListener('DOMContentLoaded', function() {
+    const tokenInput = document.getElementById('otp_token');
+    if (tokenInput) {
+        // Auto-submit when 6 digits entered (TOTP) or 8 characters (backup code)
+        tokenInput.addEventListener('input', function() {
+            const value = this.value.trim();
+            if ((value.length === 6 && /^\d{6}$/.test(value)) || 
+                (value.length === 8 && /^[a-f0-9]{8}$/i.test(value))) {
+                // Small delay to allow user to see complete input
+                setTimeout(() => {
+                    this.closest('form').submit();
+                }, 300);
+            }
+        });
+    }
+});
+</script>
+{% endblock %}