Fix broadcast_json to isolate per-connection send failures

[Copilot]

A single failing WebSocket in broadcast_json would raise and abort the entire broadcast loop, leaving dead sockets in active_connections and silently dropping messages to all other clients.

Changes

• ConnectionManager.broadcast_json: Iterate over a snapshot of active_connections and wrap each send_json in a per-connection try/except — failed sends log a warning and trigger disconnect() to evict the dead socket

async def broadcast_json(self, data) -> None:
    for connection in list(self.active_connections):
        try:
            await connection.send_json(data)
        except Exception as e:
            log.warning(f"WebSocket send failed, disconnecting client: {e}")
            await self.disconnect(connection)

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