Test an index on the last item of an ElementPath

The backend is unable to use the existing GIN index on path when looking for direct children (p.path[array_length(p."path", 1)] in Postgres, path__last in Django). The index is only designed for containment or intersection operators like @>, <@ and &&.

It might be possible to create an classic B-Tree index on p.path[array_length(p."path", 1)], either directly or via a generated column. This could make queries for direct children on ListElementChildren faster and speed up the whole navigation, as well as IIIF manifests and Create/DestroyElementParent.