feat(network): Adaptive backpressure for network event channel

[xilosada]

Problem

Network event channel has fixed capacity with no backpressure:

// crates/node/src/run.rs:94
channel_size: 1000,  // Fixed capacity

// crates/node/src/network_event_channel.rs:52
channel_size: 1000,  // Default

Impact:

• If channel fills, new events may be dropped
• No signal to slow down senders
• Burst patterns can cause loss

Proposed Solution

1. Add sender-side backpressure:

pub struct BackpressuredSender {
    inner: mpsc::Sender<NetworkEvent>,
    high_water_mark: usize,    // Start backpressure at 80%
    low_water_mark: usize,     // Release backpressure at 50%
    backpressure_active: AtomicBool,
}

impl BackpressuredSender {
    pub async fn send(&self, event: NetworkEvent) -> Result<(), SendError> {
        // Check capacity and apply backpressure delay if needed
        if len > self.high_water_mark {
            self.backpressure_active.store(true, Ordering::Relaxed);
            tokio::time::sleep(Duration::from_millis(10)).await;
        }
        self.inner.send(event).await
    }
}

2. Add metrics for channel health

Acceptance Criteria

• Backpressure activates at configurable threshold
• Sender slows down when channel near capacity
• Metrics exported for channel health
• No events dropped under normal load

Priority: P3 (Low Impact, Medium Effort)