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/stiftung/apps.py

@@ -4,3 +4,13 @@ from django.apps import AppConfig
 class StiftungConfig(AppConfig):
     default_auto_field = "django.db.models.BigAutoField"
     name = "stiftung"
+
+    def ready(self):
+        # Configure admin site with 2FA support
+        try:
+            from django_otp.admin import OTPAdminSite
+            from django.contrib import admin
+            admin.site.__class__ = OTPAdminSite
+        except ImportError:
+            # django-otp not installed
+            pass