Reservation lease time; reservations by MAC address or client ID

- A reservation can have its own lease time (--lease-time, the
  reservation dialog), over the subnet's. Schema migration 6.
- A reservation with a MAC address and a client ID matches either one
  (a dual-boot machine sends two client IDs from one MAC); before, it
  was found by MAC and then refused. A lease of the reserved address by
  the client's other identity counts as its own.
- The Leases page shows a client's client ID; reserving a lease uses
  both, or one with --by / the dialog's choice.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/e2e.sh

@@ -234,6 +234,12 @@ run "now the lowest never-used one" 0 udhcpc b 02:00:00:00:00:41
 contains "BOUND ip=$NB.10[0-8] "
 run "an unknown order is refused" 1 S subnet update iot --add-pool $NB.100-$NB.110:lowest
 
+# ---- a reservation's own lease time
+
+run "reservation lease time" 0 S reservation update $NA.20 --lease-time 2h
+run "the reserved client gets it" 0 udhcpc a 02:00:00:00:00:20
+contains "BOUND ip=$NA.20 .*lease=7200"
+
 # ---- backup
 
 run "backup" 0 docker exec ${P}-srv /w/stewardd -c /w/steward.yaml backup