FetchRSS's raw feed fails validation: entry <id> is a bare Facebook post
ID rather than a full URI, and <link rel="self"> points at the FetchRSS
URL instead of where we actually serve it. Parse the fetched XML with
DOMDocument and fix both before caching/serving: reuse each entry's own
alternate link (already a full post URL) as its id, and rewrite the self
link to our own facebook.php URL.
Requires the php-xml extension (ext-dom) — near-universal on shared
hosting, but worth checking if this breaks on deploy.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
