feat(17-01): tree reorder page and singleton admin controllers

- cabana: config_reorder.yaml (title, nameFrom, modelClass, parentColumn,
  sortColumn, maxDepth, partition) with fail-loud boot checks and the
  built-in reorder toolbar button; list schema carries reorder
- GET/POST {controller}/reorder: scoped set (ListExtendQuery plus the
  partition) locked FOR UPDATE, 409 reorder_stale on any set difference,
  422 for duplicates, foreign parents, cycles and maxDepth, both columns
  written without model hooks
- pact.AdminSingleton: GET {controller}/singleton (first or create, form
  scope checked), form schema singleton, list/create/bulk/delete 404,
  no config_list.yaml needed
- swag, inventories, token classes, CSRF walk, conformance cases, backend
  lang reorder.stale and load_failed, reorder and singleton smoke tests, docs

docs/backend/settings.md

@@ -85,6 +85,8 @@ A link entry is not a singleton and needs no embedded admin tree. [cabana](../..
 
 `GET .../settings` lists a link entry with its `controller` set (and `model` empty) only when the administrator passes the entry's permissions and may also open the controller, so a controller the administrator cannot reach is never advertised. Singleton entries carry an empty `controller`. The singleton endpoints (`.../settings/{code}`, `.../settings/{code}/schema`) answer 404 for a link entry's code.
 
+A link entry may also open a [singleton controller](admin-controllers.md#singleton-controllers), one whose `pact.AdminSingleton` record the admin opens directly in its form, the way WinterCMS plugins auto-created one definition and redirected to it. The entry stays a link: the admin asks the controller's singleton route for the record and opens it.
+
 In the admin, the entry's card on the Settings page opens the controller's pages. When the controller has no main navigation item of its own, the admin treats it as part of Settings: the rail marks Settings as current and the breadcrumbs read Settings, then the entry's label.
 
 ## How values are stored