UpdateSingleMemberField: error attribution, updated_at, snapshot newline

Attach errors to :field, :show_in_overview, :member_field_required.
Set updated_at in SQL UPDATE. Add trailing newline to snapshot JSON.

priv/resource_snapshots/repo/settings/20260223195453.json

@@ -149,4 +149,4 @@
   "repo": "Elixir.Mv.Repo",
   "schema": null,
   "table": "settings"
-}
+}