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/0014_dokumentlink_rentmeister_id.py

@@ -6,13 +6,15 @@ from django.db import migrations, models
 class Migration(migrations.Migration):
 
     dependencies = [
-        ('stiftung', '0013_alter_verwaltungskosten_status'),
+        ("stiftung", "0013_alter_verwaltungskosten_status"),
     ]
 
     operations = [
         migrations.AddField(
-            model_name='dokumentlink',
-            name='rentmeister_id',
-            field=models.UUIDField(blank=True, null=True, verbose_name='Rentmeister ID'),
+            model_name="dokumentlink",
+            name="rentmeister_id",
+            field=models.UUIDField(
+                blank=True, null=True, verbose_name="Rentmeister ID"
+            ),
         ),
     ]