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.

modules/cabana/contracts.go

@@ -131,13 +131,18 @@ func (r *Registry) Get(id string) (*CompiledController, bool) {
 	return cc, ok && cc != nil
 }
 
-// Setting returns one compiled singleton setting by stable code.
+// Setting returns one compiled singleton setting by stable code. Link entries
+// (pact.SettingsItem with Controller set) are not singletons and are not
+// returned, so the singleton settings endpoints answer 404 for their codes.
 func (r *Registry) Setting(code string) (*CompiledSetting, bool) {
 	if r == nil {
 		return nil, false
 	}
 	setting, ok := r.settings[code]
-	return setting, ok && setting != nil
+	if !ok || setting == nil || setting.Item.Controller != "" {
+		return nil, false
+	}
+	return setting, true
 }
 
 // Allows reports whether principal satisfies any of the required permission