Link previews: Open Graph tags, a social card from the record, a favicon

LinkedIn and the other unfurlers found nothing to build a preview from: the pages had a title and nothing else. Every page now carries a description, Open Graph title, description, url and image, a Twitter large-image card and a favicon. The card at web/img/og.png is the Priya and Tomás match card as the record has it: the score ring, the band, Jev's four numbers and the formula line, in the site's own palette.