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>

scripts/e2e.sh

@@ -220,6 +220,20 @@ run "delete a lease" 0 S lease delete $NA.101 --yes
 run "audit log" 0 S audit --action lease.
 contains "lease.delete"
 
+# ---- address order
+
+run "reverse address order" 0 S subnet update iot --address-order reverse
+run "the subnet shows its order" 0 S subnet show iot
+contains "addresses reverse"
+run "a new client gets the highest never-used address" 0 udhcpc b 02:00:00:00:00:40
+contains "BOUND ip=$NB.110 "
+run "a pool's own order wins" 0 S subnet update iot --add-pool $NB.100-$NB.110:sequential
+run "the pool shows it" 0 S subnet show iot
+contains "$NB.100-$NB.110:sequential"
+run "now the lowest never-used one" 0 udhcpc b 02:00:00:00:00:41
+contains "BOUND ip=$NB.10[0-8] "
+run "an unknown order is refused" 1 S subnet update iot --add-pool $NB.100-$NB.110:lowest
+
 # ---- backup
 
 run "backup" 0 docker exec ${P}-srv /w/stewardd -c /w/steward.yaml backup