[DAV] Implement unique_id method for contacts

aleksis/core/mixins.py

@@ -665,6 +665,10 @@ class ContactMixin(DAVResource, RegistryObject, is_registry=True):
     def get_dav_absolute_url(cls, reference_object, request: HttpRequest) -> str:
         return reverse("dav_resource_contact", args=["contact", cls.name, reference_object.id])
 
+    @classmethod
+    def value_unique_id(cls, reference_object, request: HttpRequest) -> str:
+        return f"{cls.name}-{reference_object.id}"
+
 
 class CalendarEventMixin(DAVResource, RegistryObject, is_registry=True):
     """Mixin for calendar feeds.