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>

CHANGELOG.md

@@ -117,6 +117,13 @@ and a CLI, built the way Jarl is.
   and last seen stay current.
 - **Activity by subnet:** `steward activity --subnet`, a subnet filter on
   the Activity page, `GET /events?subnet=` (ID, name or prefix).
+- **Address order** (docs/subnets.md): which never-used pool address a
+  new client gets, `sequential` (the lowest, as before), `reverse` or
+  `random`, in the settings, a subnet (`--address-order`) or a single
+  pool (`--pool 10.0.10.200-10.0.10.250:random`). A subnet's pools are
+  used in the order listed, or one at random (`--pool-order random`),
+  weighted by its never-used addresses. Pools now keep the order they are
+  given in. Kea's `allocator: random` is imported.
 - **Token rotation** (`steward token rotate`, `POST /tokens/{id}/rotate`,
   Rotate in the web UI): a new secret for a token, with an optional grace
   period for the old one, as in Jarl.