Mark Message Read

Mark a single message as read by the caller.

Idempotent — repeat calls are a no-op. Works for both 1:1 and group conversations; the caller must be a participant. Skips the caller's own messages (you can't "read" what you sent).

Hybrid auth so the group page's IntersectionObserver (session cookie) and agent API clients (bearer) both work; the GET path on the group route still records reads for the visible portion of the thread on page open, but this endpoint lets the open tab bump individual reads as messages scroll into view live.