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/core/__init__.py

@@ -1,4 +1,3 @@
 from .celery import app as celery
 
 __all__ = ("celery",)
-