Fix: space editors can now update shared insights in place

Space editors who didn't originally create an insight can now call update_insight successfully — version history is recorded and the insight body is updated as expected. Previously the insight_versions RLS policy blocked the version-record insert for any non-author editor, even after ticket #40 fixed the insight row itself, forcing a destructive workaround of creating a new insight and superseding the old one. Note: the workaround applied during the original incident (insight 87e9c1c5 superseded by 3d5d39ee) is not automatically reversed — the new insight is live and fully editable.