Add a public feedback board

Anyone with an account can open a suggestion, bug report or question, and anyone can vote, comment and react on it. Threads move through pending triage, backlog, in progress, done, declined or duplicate, and a roadmap view groups the open ones by status. A thread can reference another with #123 or ping someone with @handle, and once something ships its thread links straight to the changelog entry that shipped it.

This replaces the old /requests page, which is gone.