Check a Notion page before creating a site

Checks a Notion page before a site is created: resolves the canonical .notion.site URL, asks Notion whether the page is published to the web, and suggests a site name and a free .sotion.site address. No authentication.

Always 200 for a well-formed URL: an unpublished page or a link that is not a Notion page comes back with public: false, a reason (not_published, invalid_url, needs_workspace_url) and a message to relay to the user. definitive: false means Notion could not be asked; the page is let through, but tell the user the check did not complete. Results are cached for 60 seconds per page.