linking: add at-tags support to link cards master
Implements the at-tags community proposal (https://tangled.org/chrisshank.com/at-tags), which maps web pages back to atproto identities and records via <meta name="at:*"> tags in the page <head>: - VOD pages (/<handle>/video/<tid>): at:canonical points at the place.stream.video record and at:author at the streamer's DID, so clients/embedders/verifiers can attribute the page back to the VOD record. - Streamer pages (/<handle>): at:canonical points at the active place.stream.livestream record, at:author at the streamer's DID. - All pages (including the front page default card): at:me identifies the node itself via at://did:web:<BroadcasterHost>, matching the node-identity convention already used elsewhere in the repo. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>