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/kea.md

@@ -51,6 +51,7 @@ pings before it offers an address, and clients renew or rebind.
 | `shared-networks` | their subnets, all on the shared network's interface |
 | `reservations` with `hw-address` or `client-id` and `ip-address`, `hostname` | reservations |
 | `decline-probation-period` | the decline hold |
+| `allocator` (global, shared network, subnet) | `random`: the subnet's pool and address order random. `iterative` and `flq` become the default order, the lowest never-used address (with a note for `flq`) |
 | `user-context.comment` or `comment` of a subnet | the subnet's comment |
 | the lease file: active and declined leases, and released ones | leases (the last line of each address wins) |