Publish DHCP clients' names in DNS

router-webui leases-dns reads Kea's leases and publishes <name>.<domain>
with reverse records in the running Unbound (router.dns.dynamicLeases).
Names the configuration publishes are never taken over; a client's name
is cut to its first label and kept inside the local domain; of two
clients the newer lease wins. The DNS page switches it on and shows what
is published and what was skipped; router-setup enables it with a domain.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

pkgs/router-webui/collect.go

@@ -446,6 +446,7 @@ type Lease struct {
 	Address  string
 	MAC      string
 	Hostname string
+	Start    time.Time // when it was handed out or last renewed
 	Expires  time.Time
 	LAN      string
 }
@@ -514,6 +515,7 @@ func parseLeases(r io.Reader, latest map[string]Lease, now time.Time) {
 			MAC:      get(rec, "hwaddr"),
 			Hostname: strings.ReplaceAll(get(rec, "hostname"), "&#x2c", ","),
 			Expires:  time.Unix(expire, 0),
+			Start:    time.Unix(expire-lifetime, 0),
 		}
 		if lifetime == 0 || (state != "" && state != "0") {
 			l.Expires = time.Time{} // released, declined or reclaimed