Mark a thread as seen

Records that the caller has looked at this thread, which is what `GET /me/agent-runs?group=thread` compares each thread’s last activity against to decide `unread`. Idempotent, bodyless, fire-and-forget — call it when the thread view mounts and when the window regains focus. It creates nothing and starts nothing: a thread is marked seen, not read-receipted, so there is no per-message state and no notification is consumed. Authorization: whoever can read the thread’s container — its owner for a desk thread, any holder of `space:read` for a thread in a space (everyone else gets 404).