feat: federation Prometheus metrics — peer status, packets, active rooms

Wires up the existing RelayMetrics federation fields:
- wzp_federation_peer_status{peer} — 1=connected, 0=disconnected
- wzp_federation_packets_forwarded_total{peer,direction} — in/out counts
- wzp_federation_active_rooms — number of active federated rooms

These are critical for monitoring federation health and will feed into
the adaptive codec selection system (PRD-coordinated-codec.md).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/src/main.rs

@@ -392,6 +392,7 @@ async fn main() -> anyhow::Result<()> {
             room_mgr.clone(),
             endpoint.clone(),
             tls_fp.clone(),
+            metrics.clone(),
         ));
         let fm_run = fm.clone();
         tokio::spawn(async move { fm_run.run().await });