Format code with Black and isort for CI/CD compliance

- Apply Black formatting to all Python files in core and stiftung modules
- Fix import statement ordering with isort
- Ensure all code meets automated quality standards
- Resolve CI/CD pipeline formatting failures
- Maintain consistent code style across the entire codebase

app/stiftung/apps.py

@@ -1,5 +1,6 @@
 from django.apps import AppConfig
 
+
 class StiftungConfig(AppConfig):
-    default_auto_field = 'django.db.models.BigAutoField'
-    name = 'stiftung'
+    default_auto_field = "django.db.models.BigAutoField"
+    name = "stiftung"