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/0024_dokumentlink_abrechnung_id.py

@@ -6,13 +6,13 @@ from django.db import migrations, models
 class Migration(migrations.Migration):
 
     dependencies = [
-        ('stiftung', '0023_remove_legacy_verpachtung'),
+        ("stiftung", "0023_remove_legacy_verpachtung"),
     ]
 
     operations = [
         migrations.AddField(
-            model_name='dokumentlink',
-            name='abrechnung_id',
-            field=models.UUIDField(blank=True, null=True, verbose_name='Abrechnung ID'),
+            model_name="dokumentlink",
+            name="abrechnung_id",
+            field=models.UUIDField(blank=True, null=True, verbose_name="Abrechnung ID"),
         ),
     ]