Perf: avoid per-instance CustomField reload in DocumentMetadataOverrides

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.

src/documents/data_models.py

@@ -126,7 +126,7 @@ class DocumentMetadataOverrides:
         )
         overrides.custom_fields = {
             custom_field.field.id: custom_field.value
-            for custom_field in doc.custom_fields.all()
+            for custom_field in doc.custom_fields.select_related("field").all()
         }
 
         groups_with_perms = get_groups_with_perms(