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/security_coverage_test.go

@@ -44,6 +44,7 @@ var phase09Routes = []adminRoute{
 	{key: "GET /settings/{code}/schema"},
 	{key: "GET /settings/{code}"},
 	{key: "PUT /settings/{code}"},
+	{key: "POST /markdown/preview"},
 	{key: "GET /{vendor}/{plugin}/{controller}/schema/list"},
 	{key: "GET /{vendor}/{plugin}/{controller}/schema/form"},
 	{key: "GET /{vendor}/{plugin}/{controller}/schema/relation/{name}"},