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/settings.js

@@ -1,8 +1,8 @@
 // Settings: the default lease time, conflict checks, authority, and the
 // options every subnet gets.
-import { h, mount, input, field, checkbox, errorBox, readOnly } from "../ui.js";
+import { h, mount, input, field, checkbox, errorBox, readOnly, select } from "../ui.js";
 import { get, change, can } from "../api.js";
-import { optionsEditor, fmtSeconds, parseSeconds } from "../widgets.js";
+import { optionsEditor, fmtSeconds, parseSeconds, ADDRESS_ORDERS } from "../widgets.js";
 import { rerender } from "../app.js";
 
 export async function render(el) {
@@ -14,6 +14,7 @@ export async function render(el) {
   timeout.style.width = "7em";
   const hold = input({ value: fmtSeconds(s.decline_hold || 86400) });
   const auth = checkbox("Authoritative: refuse (NAK) requests for addresses that don't belong on the interface", !s.not_authoritative);
+  const order = select(ADDRESS_ORDERS, s.address_order || "sequential");
   const opts = await optionsEditor(s.options);
   const err = h("div");
   const save = async (dry) => {
@@ -22,7 +23,7 @@ export async function render(el) {
     if (lt === null || dh === null) { mount(err, h("div", { class: "msg err" }, "Write times as 12h, 1d or 30m.")); return; }
     try {
       const r = await change("PATCH", "/settings", { lease_time: lt, ping_check: ping.input.checked, ping_timeout_ms: parseInt(timeout.value, 10) || 0,
-        decline_hold: dh, not_authoritative: !auth.input.checked, options: opts.value() }, { query: dry ? { dry_run: 1 } : undefined });
+        decline_hold: dh, not_authoritative: !auth.input.checked, address_order: order.value, options: opts.value() }, { query: dry ? { dry_run: 1 } : undefined });
       if (!dry && r.applied) rerender();
     } catch (e) { mount(err, errorBox(e)); }
   };
@@ -35,6 +36,7 @@ export async function render(el) {
         field("Default lease time", lease, "How long clients keep an address unless a subnet says otherwise (12h, 1d). They renew halfway."),
         field("Options for every subnet", opts, "Usually domain-name-servers and domain-name; routers belong to each subnet."),
         field("Conflict check", h("div", null, ping, h("div", { class: "small" }, "wait ", timeout, " ms for an answer"))),
+        field("Address order", order, "Which address of a pool a new client gets: sequential (the lowest), reverse (the highest) or random. Only addresses never handed out count; when there are none left, the one free the longest. Subnets and pools can set their own."),
         field("Hold back for", hold, "How long an address a client declined (or one that answered a ping) isn't handed out."),
         field("Authority", auth, "Turn this off only if another DHCP server serves the same network.")),
       err,