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/federation.rs

@@ -129,6 +129,7 @@ pub struct FederationManager {
     room_mgr: Arc<Mutex<RoomManager>>,
     endpoint: quinn::Endpoint,
     local_tls_fp: String,
+    metrics: Arc<crate::metrics::RelayMetrics>,
     /// Active peer connections, keyed by normalized fingerprint.
     peer_links: Arc<Mutex<HashMap<String, PeerLink>>>,
     /// Dedup filter for incoming federation datagrams.
@@ -145,6 +146,7 @@ impl FederationManager {
         room_mgr: Arc<Mutex<RoomManager>>,
         endpoint: quinn::Endpoint,
         local_tls_fp: String,
+        metrics: Arc<crate::metrics::RelayMetrics>,
     ) -> Self {
         Self {
             peers,
@@ -153,6 +155,7 @@ impl FederationManager {
             room_mgr,
             endpoint,
             local_tls_fp,
+            metrics,
             peer_links: Arc::new(Mutex::new(HashMap::new())),
             dedup: Mutex::new(Deduplicator::new(DEDUP_WINDOW_SIZE)),
             rate_limiters: Mutex::new(HashMap::new()),
@@ -252,6 +255,8 @@ impl FederationManager {
             tagged.extend_from_slice(media_data);
             match link.transport.send_raw_datagram(&tagged) {
                 Ok(()) => {
+                    self.metrics.federation_packets_forwarded
+                        .with_label_values(&[&link.label, "out"]).inc();
                 }
                 Err(e) => warn!(peer = %link.label, "federation send error: {e}"),
             }
@@ -399,7 +404,8 @@ async fn run_federation_link(
     peer_fp: String,
     peer_label: String,
 ) -> Result<(), anyhow::Error> {
-    // Register peer link
+    // Register peer link + metrics
+    fm.metrics.federation_peer_status.with_label_values(&[&peer_label]).set(1);
     {
         let mut links = fm.peer_links.lock().await;
         links.insert(peer_fp.clone(), PeerLink {
@@ -483,7 +489,8 @@ async fn run_federation_link(
         _ = rtt_task => {}
     }
 
-    // Cleanup: remove peer link
+    // Cleanup: remove peer link + metrics
+    fm.metrics.federation_peer_status.with_label_values(&[&peer_label]).set(0);
     {
         let mut links = fm.peer_links.lock().await;
         links.remove(&peer_fp);
@@ -507,6 +514,11 @@ async fn handle_signal(
                 let mut links = fm.peer_links.lock().await;
                 if let Some(link) = links.get_mut(peer_fp) {
                     link.active_rooms.insert(room.clone());
+                }
+                // Update active rooms metric
+                let total: usize = links.values().map(|l| l.active_rooms.len()).sum();
+                fm.metrics.federation_active_rooms.set(total as i64);
+                if let Some(link) = links.get_mut(peer_fp) {
                     link.remote_participants.insert(room.clone(), participants.clone());
                 }
                 // Propagate to other peers
@@ -596,6 +608,10 @@ async fn handle_datagram(
         None => return,
     };
 
+    // Count inbound federation packet
+    fm.metrics.federation_packets_forwarded
+        .with_label_values(&[source_peer_fp, "in"]).inc();
+
     // Dedup: drop packets we've already seen (multi-path duplicates)
     {
         let mut dedup = fm.dedup.lock().await;