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/migrations/0023_remove_legacy_verpachtung.py

@@ -6,11 +6,11 @@ from django.db import migrations
 class Migration(migrations.Migration):
 
     dependencies = [
-        ('stiftung', '0022_dokumentlink_land_verpachtung_id_and_more'),
+        ("stiftung", "0022_dokumentlink_land_verpachtung_id_and_more"),
     ]
 
     operations = [
         migrations.DeleteModel(
-            name='Verpachtung',
+            name="Verpachtung",
         ),
     ]