Set due date on invoice

aleksis/apps/paweljong/models.py

-from datetime import datetime
+from datetime import datetime, timedelta
 from decimal import Decimal
 
 from django.contrib.contenttypes.models import ContentType
@@ -350,6 +350,7 @@ class EventRegistration(ExtensibleModel):
                 "number": f"HNF-{self.date_registered.strftime('%Y-%m')}-{self.id}",
                 "currency": "EUR",
                 "total": self._get_total_amount()[0],
+                "due_date": now().date() + timedelta(days=7),
                 "tax": self._get_total_amount()[1],
                 "description": _("Participation of {} in event {}").format(
                     self.person.addressing_name, self.event.display_name