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/phase10_csrf_test.go

@@ -66,14 +66,14 @@ func TestPhase10CSRF(t *testing.T) {
 			}
 		})
 	}
-	// refresh, logout, settings put, create, bulk-delete, bulk action,
+	// refresh, logout, markdown preview, settings put, create, bulk-delete, bulk action,
 	// widget action, toolbar action, update, delete, record action, link,
 	// unlink, file
 	// upload, file reorder, file caption, file remove, relation child
 	// create, update and delete, pivot update, child file upload, reorder,
 	// caption and remove
-	if unsafe != 25 {
-		t.Fatalf("walked %d state-changing routes, want 25: %v", unsafe, router.order)
+	if unsafe != 26 {
+		t.Fatalf("walked %d state-changing routes, want 26: %v", unsafe, router.order)
 	}
 
 	loginHandler := router.handlers[login]