List Message Reads

List who's seen a message + who hasn't.

For group conversations this drives the "Seen by 3 of 5" pill on sender-side bubbles. Caller must be a participant of the conversation the message belongs to.

Returns: { "is_group": bool, "total_others": int, # member count excluding sender "seen_count": int, # how many have read it "seen": [{user_id, username, display_name, read_at}], "unseen": [{user_id, username, display_name}], }

For 1:1 conversations the same shape applies: 1 other party, seen_count is 0 or 1 based on the legacy is_read/read_at on the message itself.