WebSub for subscribers and publishers: find the hubs of a topic, ask a
hub for a subscription, answer the hub at the callback URL, and tell the
hubs when a topic changes.

The code follows the W3C Recommendation of 2 June 2026 at
https://www.w3.org/TR/websub/ and cites its sections. A subscriber is a
server with a public URL, since the hub calls its callback. The hub
itself is in dk.simongray.websub.hub.

A WebSub hub: it takes subscriptions, checks each with its subscriber,
and delivers each new version of a topic to the topic's subscribers.

Nothing runs in the background. The Ring handler answers subscribers and
publishers at once, and gives the work that follows to the :schedule!
function of the options as a job, a map that run-job! does. A delivery
that fails gives its retry to :schedule! in the same way, with the
instant to run it as its :at. The subscriptions are in a store of yours,
a map of two functions, or in memory-store.

The code follows sections 5 to 8 of the W3C Recommendation of 2 June
2026, https://www.w3.org/TR/websub/.