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>

internal/server/h_config.go

@@ -440,6 +440,9 @@ func applyReservationInput(res *model.Reservation, in *api.ReservationInput) err
 		}
 		res.Options = opts
 	}
+	if in.LeaseTime != nil {
+		res.LeaseTime = *in.LeaseTime
+	}
 	if in.Disabled != nil {
 		res.Disabled = *in.Disabled
 	}