Add host-mode heartbeat: vetting-agent host + last-seen badge

vetting-agent gains a `host` subcommand that runs as a systemd service
installed by the quick-register one-liner, POSTing every 30s to
/api/v1/hosts/{mac}/heartbeat so the dashboard tile shows "online" or
"Nm ago" without waiting on WoL. Ships dormant client code for the
Phase 2 reboot_for_vetting command so the server can flip it on later
without a binary redeploy.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/db/migrations/0002_add_hosts_last_seen_at.sql

@@ -0,0 +1,2 @@
+-- Persistent host-mode agent heartbeats land here. NULL = never seen.
+ALTER TABLE hosts ADD COLUMN last_seen_at TIMESTAMP;