feat: add a ticket activity feed

todo.sr.ht's root events query returns what the authenticated user is
subscribed to or implicated in, newest first, across every tracker including
ones they do not own. It was never called, so the only way to notice a reply was
to open the ticket.

Reuses EventChange, which already decodes todo's polymorphic EventDetail for
ticket timelines, so the same inline fragments describe both surfaces. Rows push
straight to the ticket.

events is nullable and comes back null when the token lacks the EVENTS scope, so
that case reports a scope problem rather than an empty feed, which would read as
"nothing has happened".

Paginates rather than fetching everything: an active account's history is
unbounded and the top of it is the whole point.

archiveMessage, the other half of this roadmap item, is not here. It is marked
@internal in the schema and inaccessible, like revokePersonalAccessToken already
recorded in SCOPE.md.

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