Hidden interfaces, last-seen pings, activity by subnet

- interfaces.exclude in steward.yaml (names or shell patterns) leaves
  interfaces out of GET /interfaces, the web UI's picker and
  'steward interface list'. Subnets on them are still served.
- With the ping check on, active leases are pinged every minute; a
  client that answers gets a last-seen time (leases.last_seen,
  migration 4). Not answering never takes a client offline.
- The activity log filters by subnet (--subnet, the Activity page,
  GET /events?subnet=).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/dhcpd/handle_test.go

@@ -461,3 +461,41 @@ func TestDisabledReservation(t *testing.T) {
 		t.Errorf("the printer got %s from a full pool", out.Packet.YIAddr)
 	}
 }
+
+func TestLastSeen(t *testing.T) {
+	h := newHarness(t, nil)
+	up := h.lease("eth0", mac(1)).YIAddr
+	quiet := h.lease("eth0", mac(2)).YIAddr
+	h.ping.answers[up] = true
+	h.s.CheckSeen(context.Background())
+	if l := h.s.Lease(up); l.LastSeen == nil || !l.LastSeen.Equal(h.clock) {
+		t.Errorf("answering client: %+v", l)
+	}
+	// One that never answers has no last-seen time, and stays active.
+	if l := h.s.Lease(quiet); l.LastSeen != nil || !l.ActiveAt(h.clock) {
+		t.Errorf("quiet client: %+v", l)
+	}
+	// Later, the time stays when it stops answering, and survives a
+	// renewal and a restart.
+	seen := h.clock
+	h.clock = h.clock.Add(10 * time.Minute)
+	h.ping.answers[up] = false
+	h.s.CheckSeen(context.Background())
+	h.lease("eth0", mac(1))
+	if l := h.s.Lease(up); l.LastSeen == nil || !l.LastSeen.Equal(seen) {
+		t.Errorf("after a renewal: %+v", l)
+	}
+	ls, _ := h.st.LoadLeases(context.Background())
+	for _, l := range ls {
+		if l.IP == up && (l.LastSeen == nil || !l.LastSeen.Equal(seen)) {
+			t.Errorf("stored: %+v", l)
+		}
+	}
+	// Expired leases aren't pinged.
+	h.clock = h.clock.Add(48 * time.Hour)
+	h.ping.pinged = nil
+	h.s.CheckSeen(context.Background())
+	if len(h.ping.pinged) != 0 {
+		t.Errorf("pinged %v", h.ping.pinged)
+	}
+}