feat(cabana): add markdown preview admin route

- POST {prefix}/api/v1/markdown/preview renders {markdown} through
  cabana.RenderMarkdown in the backend-guarded group behind requireAjax
- refused output is a 422 validation_failed on markdown with a fixed message
- swag annotation, regenerated admin.json and schema.d.ts
- route inventories, CSRF walk (26) and OpenAPI conformance learn the route
- README and docs/backend/forms.md document the route

modules/cabana/http.go

@@ -237,6 +237,8 @@ func (s *service) mount(r pact.Router) {
 	r.GroupRaw(api, []string{"backend"}, func(g pact.Router) {
 		g.Get("/auth/me", s.me)
 		g.Get("/navigation", s.navigation)
+		// Form previews render through RenderMarkdown; nothing is stored.
+		g.Post("/markdown/preview", requireAjax(s.markdownPreview))
 		g.Get("/settings", s.settingsList)
 		g.Get("/settings/{code}/schema", s.settingsSchema)
 		constrainSetting(g)