Module stream 

Phase 2: Firebase RTDB Server-Sent Events (SSE) streaming.

Will use eventsource-client to subscribe to real-time device state changes, eliminating the need for polling.

Planned API:

let stream = client.watch_device(device_id).await?;
while let Some(event) = stream.next().await {
    match event? {
        DeviceEvent::StateChanged(data) => { /* full state */ }
        DeviceEvent::FieldUpdated(path, value) => { /* partial update */ }
        DeviceEvent::KeepAlive => {}
    }
}