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/server.go

@@ -10,7 +10,6 @@ import (
 	"net"
 	"net/netip"
 	"sort"
-	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -24,6 +23,7 @@ type LeaseStore interface {
 	PutLease(ctx context.Context, l *model.Lease) error
 	DeleteLease(ctx context.Context, ip netip.Addr) error
 	PruneLeases(ctx context.Context, before time.Time) (int64, error)
+	SetLastSeen(ctx context.Context, seen map[netip.Addr]time.Time) error
 }
 
 // Interfaces reports a network interface's IPv4 addresses with their
@@ -32,7 +32,8 @@ type Interfaces interface {
 	Addrs(name string) ([]netip.Prefix, error)
 }
 
-// Pinger checks whether an address answers (conflict detection).
+// Pinger checks whether an address answers (conflict detection, and
+// clients' last-seen times).
 type Pinger interface {
 	Ping(ctx context.Context, a netip.Addr, timeout time.Duration) bool
 }
@@ -342,15 +343,15 @@ func (e *events) add(ev Event) {
 	e.counts[ev.Type]++
 }
 
-// Events returns the latest events, newest first, optionally only those of
-// one MAC address or interface.
-func (s *Server) Events(limit int, mac, iface string) []Event {
+// Events returns the latest events, newest first, optionally only those
+// keep accepts.
+func (s *Server) Events(limit int, keep func(Event) bool) []Event {
 	s.events.mu.Lock()
 	defer s.events.mu.Unlock()
 	out := []Event{}
 	for i := len(s.events.list) - 1; i >= 0 && (limit <= 0 || len(out) < limit); i-- {
 		ev := s.events.list[i]
-		if mac != "" && !strings.EqualFold(ev.MAC, mac) || iface != "" && ev.Interface != iface {
+		if keep != nil && !keep(ev) {
 			continue
 		}
 		out = append(out, ev)