dashboard gains an admin-only queues block: per worker (webhooks, mail, mirrors, builds, deps) the pending, retrying, and dead-lettered counts, oldest pending age, and the retrying or failed rows themselves, capped at twenty each. /admin renders it by dispatching dashboard; non-admins 404 and see no rail link.
The e2e test drives a failing webhook receiver and a runner-less CI job and asserts both surfaces, and that a non-admin's dashboard carries no queues key.
Wiki: Admin page updated, pushed with the merge.
Closes #74