Fix beacon TTL reaping too aggressively during reconnection

Increased beacon node TTL from 5 to 10 minutes so nodes survive
registry outages without losing beacon registration. Also fixed a
nil pointer dereference in handlePunchRequest when the requester
is unknown (beacon at capacity).

Author: teovl

pkg/beacon/server.go

@@ -59,7 +59,9 @@ const maxRelayPayload = 65535
 const maxBeaconNodes = 100_000
 
 // beaconNodeTTL is how long a node entry lives without a discover refresh.
-const beaconNodeTTL = 5 * time.Minute
+// Set to 10 minutes (well above the 60s heartbeat-driven re-discover interval)
+// so nodes survive brief registry outages without losing beacon registration.
+const beaconNodeTTL = 10 * time.Minute
 
 func New() *Server {
 	return NewWithPeers(0, nil)
@@ -267,6 +269,10 @@ func (s *Server) handlePunchRequest(data []byte, remote *net.UDPAddr) {
 		slog.Warn("punch target not found", "target_id", targetID)
 		return
 	}
+	if requesterNode == nil {
+		slog.Warn("punch requester not found", "requester_id", requesterID)
+		return
+	}
 
 	targetAddr := targetNode.addr
 	requesterAddr := requesterNode.addr