feat(cabana): settings entries that link to an admin controller

pact.SettingsItem gains an additive Controller field, the equivalent of a
WinterCMS registerSettings 'url' => Backend::url(...) entry. A link entry
declares no Model, Form or NewModel and needs no AdminFS; start-up fails
when it combines Controller with a singleton form or a model, or names an
unregistered controller. Settings codes stay one namespace.

GET /settings lists a link entry with its controller only when the
principal passes the item's permissions and may open the controller.
Registry.Setting never returns a link entry, so the singleton settings
endpoints answer 404 for its code. SettingsEntry carries controller,
empty for singletons; the OpenAPI document, generated SPA types and
settings fixture follow, and the pact and cabana READMEs and the
settings docs describe the link.

admin/src/api/schema.d.ts

@@ -4886,6 +4886,11 @@ export interface components {
         "cabana.SettingsEntry": {
             category: string;
             code: string;
+            /**
+             * @description Controller is the admin controller a link entry opens; empty for a
+             *     singleton settings form.
+             */
+            controller: string;
             description: string;
             icon: string;
             keywords: string[];