send_websocket_document_updated calls document.refresh_from_db()
before building overrides, which drops the custom_fields prefetch
(and its select_related("field")) set up by the view's queryset.
DocumentMetadataOverrides.from_document() then lazily reloads field
once per custom field instance. Since from_document() can't rely on
the caller having a prefetched document, select_related explicitly at
the point of use instead.
