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/0024_dokumentlink_abrechnung_id.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2025-08-31 21:22
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('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'),
+        ),
+    ]