Expand description
§Open Graph / link-preview extraction
Parses the <head> of an HTML page and extracts the fields needed to render a link
preview card in a post: title, description, image, and canonical URL.
Open Graph (og:title, og:description, og:image, og:url) is preferred; the
extractor falls back to the page’s <title> element and <meta name="description">
when OG isn’t present. The UrlPreviewData struct is what callers hand up to the
protocol layer — servers fetch the target URL under an RPC budget gated by
POW_MINIMUM_PER_URL_FETCH and return this struct back to the client so the preview
card can render without every client individually fetching (and thus leaking its IP to)
the target site.