1. WS disconnect now unsubscribes from user_notification_inner.
Previously, every WebSocket connection created a broadcast channel
for user notifications that was never removed on disconnect, causing
unbounded growth proportional to unique connected users over time.
2. Room worker tasks now use the manager's room_shutdown_txs channel
instead of a local broadcast channel. shutdown_room() sends on this
channel, so when a room is deleted the worker task receives the signal
and terminates, releasing its DashMap (capacity 10,000) and all
captured closures. Previously the worker ran forever.
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