Mark Channel Read

The reader says "I have seen this channel up to now."

Until this existed, a channel was marked read only as a side effect of GET /messages, so a message that ARRIVED over the socket while the channel was open was never marked read: the list kept a badge on the very channel the person was looking at until they refreshed (Nancy, 2026-09-11). Direct messages have had PUT /chat/mark-read for this since the unread counters were denormalised; this is the channel twin. Idempotent; a non-member is a no-op rather than an error, because the client calls it on every incoming message and a stale membership must not turn into a red banner.