Image upload endpoint for Help Center articles

Provide a way to attach images to Help Center articles through the API, ideally a multipart upload endpoint that stores the file and returns a Featurebase-hosted URL for use in the article body.

Alternatively, add multipart support to articles.create and articles.update, or confirm that base64 data URIs and external URLs in an article body are pulled into Featurebase storage as the changelog docs describe.

Local screenshot files have no public URL, and embedding base64 bytes in the tool call is slow, error-prone and bloats the body, so the final step currently has to be done manually in the dashboard editor.