feat(19.3-04): pact.ReorderAfterSave hook inside cabana's reorder transaction

- optional ReorderAfterSave(ctx, partition) runs after the new order is
  written; an error rolls the reorder back through lifecycleFailure
- reorderPost runs in lagoon.Transaction so lagoon.AfterCommit work a hook
  registers (a site-cache purge) runs only after the commit
- pact and cabana READMEs, admin hook table, tree reorder docs

docs/services/caching.md

@@ -156,7 +156,7 @@ func (c articlesController) FormAfterUpdate(ctx context.Context, model any) erro
 }
 ```
 
-A nil purger (no purge configured) does nothing, so the hooks need no configuration check.
+A nil purger (no purge configured) does nothing, so the hooks need no configuration check. A tree or sortable list's reorder save runs `pact.ReorderAfterSave` the same way; see [Tree reorder](../backend/lists-and-filters.md#tree-reorder).
 
 ### The site side