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/store/leases.go

@@ -2,13 +2,14 @@ package store
 
 import (
 	"context"
+	"database/sql"
 	"net/netip"
 	"time"
 
 	"steward/internal/model"
 )
 
-const leaseCols = `ip, subnet_id, mac, client_id, hostname, state, start, end, interface, updated`
+const leaseCols = `ip, subnet_id, mac, client_id, hostname, state, start, end, interface, updated, last_seen`
 
 // LoadLeases reads every lease.
 func (s *Store) LoadLeases(ctx context.Context) ([]*model.Lease, error) {
@@ -21,11 +22,13 @@ func (s *Store) LoadLeases(ctx context.Context) ([]*model.Lease, error) {
 	for rows.Next() {
 		l := &model.Lease{}
 		var ip, start, end, updated string
-		if err := rows.Scan(&ip, &l.SubnetID, &l.MAC, &l.ClientID, &l.Hostname, &l.State, &start, &end, &l.Interface, &updated); err != nil {
+		var seen sql.NullString
+		if err := rows.Scan(&ip, &l.SubnetID, &l.MAC, &l.ClientID, &l.Hostname, &l.State, &start, &end, &l.Interface, &updated, &seen); err != nil {
 			return nil, err
 		}
 		l.IP, _ = netip.ParseAddr(ip)
 		l.Start, l.End, l.Updated = parseTS(start), parseTS(end), parseTS(updated)
+		l.LastSeen = parseTSPtr(seen)
 		out = append(out, l)
 	}
 	return out, rows.Err()
@@ -46,10 +49,26 @@ func (s *Store) PutLeases(ctx context.Context, ls []*model.Lease) error {
 	}
 	defer tx.Rollback()
 	for _, l := range ls {
-		if _, err := tx.ExecContext(ctx, `INSERT INTO leases (`+leaseCols+`) VALUES (?,?,?,?,?,?,?,?,?,?)
+		if _, err := tx.ExecContext(ctx, `INSERT INTO leases (`+leaseCols+`) VALUES (?,?,?,?,?,?,?,?,?,?,?)
 			ON CONFLICT(ip) DO UPDATE SET subnet_id = excluded.subnet_id, mac = excluded.mac, client_id = excluded.client_id, hostname = excluded.hostname,
-			state = excluded.state, start = excluded.start, end = excluded.end, interface = excluded.interface, updated = excluded.updated`,
-			l.IP.String(), l.SubnetID, l.MAC, l.ClientID, l.Hostname, l.State, ts(l.Start), ts(l.End), l.Interface, ts(l.Updated)); err != nil {
+			state = excluded.state, start = excluded.start, end = excluded.end, interface = excluded.interface, updated = excluded.updated, last_seen = excluded.last_seen`,
+			l.IP.String(), l.SubnetID, l.MAC, l.ClientID, l.Hostname, l.State, ts(l.Start), ts(l.End), l.Interface, ts(l.Updated), tsPtr(l.LastSeen)); err != nil {
+			return err
+		}
+	}
+	return tx.Commit()
+}
+
+// SetLastSeen records when leases' clients answered a ping, in one
+// transaction.
+func (s *Store) SetLastSeen(ctx context.Context, seen map[netip.Addr]time.Time) error {
+	tx, err := s.db.BeginTx(ctx, nil)
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+	for ip, t := range seen {
+		if _, err := tx.ExecContext(ctx, `UPDATE leases SET last_seen = ? WHERE ip = ?`, ts(t), ip.String()); err != nil {
 			return err
 		}
 	}