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>

internal/web/static/assets/pages/subnets.js

@@ -1,12 +1,12 @@
 // The subnets, and the New subnet dialog.
 import { h, mount, table, modal, errorBox, badge } from "../ui.js";
 import { get, change, can } from "../api.js";
-import { usageBar, subnetForm, fmtSeconds, subnetLabel } from "../widgets.js";
+import { usageBar, subnetForm, fmtSeconds, subnetLabel, poolText } from "../widgets.js";
 import { toggleSubnet } from "./subnet.js";
 
 export async function render(el, params, query) {
   const subnets = await get("/subnets");
-  const poolsText = (s) => s.pools.length ? s.pools.map((p) => p.start + "-" + p.end).join(", ") : "–";
+  const poolsText = (s) => s.pools.length ? s.pools.map(poolText).join(", ") : "–";
   mount(el,
     h("div", { class: "page-head" },
       h("div", { class: "grow" }, h("h1", null, "Subnets"),