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/store.go

@@ -197,6 +197,10 @@ ALTER TABLE tokens ADD COLUMN prev_expires_at TEXT;
 	// 3: disabled reservations.
 	`
 ALTER TABLE reservations ADD COLUMN disabled INTEGER NOT NULL DEFAULT 0;
+`,
+	// 4: when a lease's client last answered a ping.
+	`
+ALTER TABLE leases ADD COLUMN last_seen TEXT;
 `,
 }