fix: configure CI database connection properly

- Add dotenv loading to Django settings
- Update CI workflow to use correct environment variables
- Set POSTGRES_* variables instead of DATABASE_URL
- Add environment variables to all Django management commands
- Fixes CI test failures due to database connection issues

app/stiftung/migrations/0023_remove_legacy_verpachtung.py

@@ -0,0 +1,16 @@
+# Generated by Django 5.0.6 on 2025-08-31 20:25
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0022_dokumentlink_land_verpachtung_id_and_more'),
+    ]
+
+    operations = [
+        migrations.DeleteModel(
+            name='Verpachtung',
+        ),
+    ]