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/admin_openapi.go

@@ -245,6 +245,24 @@ func AdminSettingsGet() {}
 // @Router /settings/{code} [put]
 func AdminSettingsPut() {}
 
+// AdminMarkdownPreview documents POST /markdown/preview.
+//
+// @Summary Render a markdown preview
+// @Description Renders the markdown source through cabana.RenderMarkdown (goldmark without unsafe HTML) and answers the sanitized HTML. Output the renderer's gate refuses is a 422 on markdown. Any backend session may call it.
+// @Tags admin
+// @Accept json
+// @Produce json
+// @Security BackendBearer
+// @Param body body AdminMarkdownPreviewRequest true "Markdown source"
+// @Success 200 {object} Envelope[AdminMarkdownPreviewResult]
+// @Failure 401 {object} ErrorEnvelope
+// @Failure 403 {object} ErrorEnvelope
+// @Failure 404 {object} ErrorEnvelope
+// @Failure 413 {object} ErrorEnvelope
+// @Failure 422 {object} ErrorEnvelope
+// @Router /markdown/preview [post]
+func AdminMarkdownPreview() {}
+
 // AdminListSchema documents the list schema route.
 //
 // @Summary Admin list schema