Add Room Capacity Limit Feature

[bibekjoshi01]

Add a feature to allow creators (or system) to define a maximum number of participants per chat room. This will enforce a cap on how many users can join a room simultaneously.

Goals

1. Allow the room to have a capacity value (e.g. 2, 5, 10 users)
2. Store and check the capacity when someone attempts to join
3. Deny new WebSocket connections if the room is full
4. Send a proper system message like:
5. "Room is full. You cannot join."
6. Show current member count on the UI (e.g., 3/5 users)