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>

docs/subnets.md

@@ -58,15 +58,20 @@ steward reservation add 10.0.10.20 aa:bb:cc:dd:ee:01 --hostname nas
 steward lease reserve 10.0.10.57       # the address a client has now
 ```
 
-* **The client** is known by its MAC address, or by its client identifier
-  (option 61) when the reservation has one (`--client-id`; give `-` as the
-  MAC). Some devices change their identifier but not their MAC address,
-  and others the other way round. When unsure, use the MAC address.
+* **The client** is known by its MAC address, its client identifier
+  (option 61, `--client-id`; give `-` as the MAC for an identifier alone),
+  or both. With both, **either one** gets the client its address: a
+  machine that boots two systems sends different identifiers from one MAC
+  address, and a device with a randomised MAC may keep its identifier.
+  The Leases page and `steward lease show` show the identifier a client
+  sent; `steward lease reserve IP` uses both when it sent one (`--by mac`
+  or `--by client-id` for just one).
 * **The address picks the subnet**: the subnet whose network holds it. It
   may be inside a pool or outside one; either way, nobody else gets it.
 * **The host name** is sent to the client (option 12).
 * **Options** set on the reservation are for that client only, and win
   over the subnet's and the settings'.
+* **A lease time** of its own (`--lease-time 7d`) wins over the subnet's.
 * **A device on two VLANs** can have a reservation in each subnet. Within
   one subnet, a client has at most one.
 
@@ -108,7 +113,9 @@ a list, it gets every option set for it.
 ## Lease times
 
 The settings have the default (12 hours, `steward settings set
---lease-time 1d`); a subnet can have its own (`--lease-time 1h`). Clients
+--lease-time 1d`); a subnet can have its own (`--lease-time 1h`), and a
+reservation its own over that (`steward reservation update nas
+--lease-time 7d`). Clients
 renew halfway through. Short times let changes reach clients sooner;
 long ones keep clients working longer if Steward is down.