Address order: sequential, reverse or random, per subnet and pool

- Which never-used pool address a new client gets: sequential (the
  lowest, as before), reverse or random, in the settings, a subnet or
  a single pool (FIRST-LAST:ORDER). When no never-used address is left,
  the one free the longest, whatever the order; returning clients keep
  theirs.
- A subnet's pools are used in the order listed, or one at random
  weighted by its never-used addresses. Pools now keep the order they
  are given in (no more sorting by address).
- CLI --address-order and --pool-order; --add-pool of an existing range
  changes its order in place. Web UI: per-pool order and a move-up
  button, pool and address order on the subnet and settings pages.
- Kea's allocator: random is imported. Schema migration 5.

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

docs/validation.md

@@ -17,6 +17,7 @@ interfaces. They come from stewardd, not from a configuration on its own.
 | `lease_time_range` | error | a lease time (settings or subnet) under 1 minute or over 365 days |
 | `ping_timeout_range` | error | the ping timeout isn't 50 to 5000 ms |
 | `decline_hold_range` | error | the decline hold isn't 1 minute to 30 days |
+| `address_order` | error | an address order (settings, subnet or pool) that isn't `sequential`, `reverse` or `random` |
 
 ## Options
 
@@ -45,6 +46,7 @@ These apply to options anywhere (settings, subnets, reservations).
 | `pool_outside_subnet` | error | a pool not inside the subnet |
 | `pool_network_address` | error | a pool with the network or broadcast address |
 | `pool_overlap` | error | two pools overlap |
+| `pool_order` | error | a pool order that isn't `listed` or `random` |
 | `router_outside_subnet` | warning | a `routers` address outside the subnet: clients can't reach it |
 | `router_in_pool` | warning | a `routers` address inside a pool. The ping check keeps it from being handed out while it answers, but take it out of the pool |
 | `interface_unknown` | warning, host | the interface doesn't exist on this machine. It is served once it appears |