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>

docs/design.md

@@ -25,7 +25,9 @@ check or reload.
 * **Leases** in SQLite, written before the client is answered, so a crash
   or restart never hands out an address twice.
 * **Conflict checks**: an address is pinged before it is offered; one that
-  answers, or that a client declines, is held back for a while.
+  answers, or that a client declines, is held back for a while. Active
+  leases are pinged every minute too, for a "last seen" time that never
+  takes a client offline ([subnets.md](subnets.md#online-and-last-seen)).
 * **The Jarl way of working**: an API used by both the CLI and the web UI;
   users, sessions and API tokens; every configuration change validated as a
   whole before it is saved and used; a history of changes that can be