Feature: Add per-message read count and reader list

Summary

Add the ability for users to see how many room members have read each message and who they are, providing visibility into message engagement within a channel.

Motivation

In team communication, it's often important to know whether a message has been seen by the intended recipients — especially for announcements, action items, or time-sensitive information. Currently there's no
way to tell if a specific message was read or by whom.

Proposed Behavior

• A read count badge is displayed on each message indicating how many members have read it
• The count updates in real-time as members read the message
• Hovering over the badge shows a tooltip with the list of users who have read the message
• Read status is determined by the room's subscription lastSeen timestamp relative to the message timestamp

Start by locating the message rendering entry point and the room subscription data that provides each member's lastSeen timestamp. Define how the count, real-time updates, and reader-list tooltip should use message timestamps; done means each message shows the correct count and reader list as members read it.