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

modules/pact/capabilities.go

@@ -765,6 +765,15 @@ type FormAfterDelete interface {
 	FormAfterDelete(ctx context.Context, model any) error
 }
 
+// ReorderAfterSave optionally runs after a reorder save (POST .../reorder of
+// a tree or sortable list) wrote the new order, inside the same transaction,
+// so cabana.TxFromContext returns it and lagoon.AfterCommit work waits for
+// the commit. partition is the reorder request's partition (nil when the
+// list has none). A returned error rolls the reorder back.
+type ReorderAfterSave interface {
+	ReorderAfterSave(ctx context.Context, partition *uint64) error
+}
+
 // FormVirtualFields optionally names fields of the controller's form that are
 // not model columns for the purpose of the form, such as a password and its
 // confirmation or a "send an invitation" checkbox. The admin framework never