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/0028_alter_helpbox_page_key.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2025-09-02 20:53
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stiftung', '0027_helpbox_alter_appconfiguration_category'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='helpbox',
+            name='page_key',
+            field=models.CharField(choices=[('destinataer_new', 'Neuer Destinatär'), ('unterstuetzung_new', 'Neue Unterstützung'), ('foerderung_new', 'Neue Förderung'), ('paechter_new', 'Neuer Pächter'), ('laenderei_new', 'Neue Länderei'), ('verpachtung_new', 'Neue Verpachtung'), ('land_abrechnung_new', 'Neue Landabrechnung'), ('person_new', 'Neue Person'), ('konto_new', 'Neues Konto'), ('verwaltungskosten_new', 'Neue Verwaltungskosten'), ('rentmeister_new', 'Neuer Rentmeister'), ('dokument_new', 'Neues Dokument'), ('user_new', 'Neuer Benutzer'), ('csv_import_new', 'CSV Import'), ('destinataer_notiz_new', 'Destinatär Notiz')], max_length=50, unique=True, verbose_name='Seite'),
+        ),
+    ]