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/store/store.go

@@ -201,6 +201,11 @@ ALTER TABLE reservations ADD COLUMN disabled INTEGER NOT NULL DEFAULT 0;
 	// 4: when a lease's client last answered a ping.
 	`
 ALTER TABLE leases ADD COLUMN last_seen TEXT;
+`,
+	// 5: the order addresses are handed out in (a pool's own is in pools).
+	`
+ALTER TABLE subnets ADD COLUMN pool_order TEXT NOT NULL DEFAULT '';
+ALTER TABLE subnets ADD COLUMN address_order TEXT NOT NULL DEFAULT '';
 `,
 }