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>

CHANGELOG.md

@@ -124,6 +124,12 @@ and a CLI, built the way Jarl is.
   used in the order listed, or one at random (`--pool-order random`),
   weighted by its never-used addresses. Pools now keep the order they are
   given in. Kea's `allocator: random` is imported.
+- **A reservation's own lease time** (`--lease-time 7d`, the reservation
+  dialog), over the subnet's.
+- **Reservations by MAC address and client ID together**: either one gets
+  the client its address (a dual-boot machine sends two client IDs from
+  one MAC). The Leases page shows the client ID a client sent, and
+  reserving a lease uses both (`steward lease reserve --by` for one).
 - **Token rotation** (`steward token rotate`, `POST /tokens/{id}/rotate`,
   Rotate in the web UI): a new secret for a token, with an optional grace
   period for the old one, as in Jarl.