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/model/model.go

@@ -14,6 +14,7 @@ type Host struct {
 	Notes            string
 	CreatedAt        time.Time
 	UpdatedAt        time.Time
+	LastSeenAt       *time.Time // host-mode agent heartbeat; nil = never seen
 }
 
 type RunState string