AI Composer workflow for batch image download and replace

Backend: Add a new endpoint /api/download-and-save-image. Accept an image URL, download it, save to object storage (R2). File naming rule: 'date + original filename', if duplicate, append a number suffix. Return the new image URL. Reference existing /api/upload-image code for structure and style. Frontend: Use React + TypeScript + ShadcnUI, editor is TipTap. Generate a component with: a button + progress bar or status message; on click, traverse all images in the editor (editor.getJSON()), filter out links starting with http:// or https://; call the new backend endpoint to batch download and upload images; replace successfully uploaded image links back into the editor and update the progress bar. Provide the backend request/response format: POST method, Body is { url: string }, success returns new image URL, failure returns error message.