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/store/store.go

@@ -206,6 +206,10 @@ ALTER TABLE leases ADD COLUMN last_seen TEXT;
 	`
 ALTER TABLE subnets ADD COLUMN pool_order TEXT NOT NULL DEFAULT '';
 ALTER TABLE subnets ADD COLUMN address_order TEXT NOT NULL DEFAULT '';
+`,
+	// 6: a reservation's own lease time.
+	`
+ALTER TABLE reservations ADD COLUMN lease_time INTEGER NOT NULL DEFAULT 0;
 `,
 }