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/openapi_conformance_test.go

@@ -160,6 +160,9 @@ func TestPhase10OpenAPIConformance(t *testing.T) {
 		{"PUT /settings/{code}", 200, "cabana.Envelope-cabana_SettingsResult", func(t *testing.T, e *conformEnv) *httptest.ResponseRecorder {
 			return e.send(t, http.MethodPut, "/settings/conform", map[string]any{"enabled": true}, true)
 		}, into[cabana.Envelope[cabana.SettingsResult]](), nil},
+		{"POST /markdown/preview", 200, "cabana.Envelope-cabana_AdminMarkdownPreviewResult", func(t *testing.T, e *conformEnv) *httptest.ResponseRecorder {
+			return e.send(t, http.MethodPost, "/markdown/preview", map[string]string{"markdown": "# Conform"}, true)
+		}, into[cabana.Envelope[cabana.AdminMarkdownPreviewResult]](), nil},
 		{"GET /{vendor}/{plugin}/{controller}/schema/list", 200, "cabana.Envelope-cabana_ListSchema", func(t *testing.T, e *conformEnv) *httptest.ResponseRecorder {
 			rec := e.send(t, http.MethodGet, "/acme/conform/gadgets/schema/list", nil, true)
 			var body cabana.Envelope[cabana.ListSchema]