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/server/server_test.go

@@ -405,3 +405,32 @@ func TestInterfacesExclude(t *testing.T) {
 		t.Errorf("interfaces: %+v", ifs)
 	}
 }
+
+func TestAddressOrderAPI(t *testing.T) {
+	e := newEnv(t)
+	var mr api.MutationResponse
+	in := api.SubnetInput{Prefix: ptr("10.0.20.0/24"), Interface: ptr("eth0.20"), PoolOrder: ptr("random"), AddressOrder: ptr("Reverse"),
+		Pools: &[]string{"10.0.20.100-10.0.20.110:random", "10.0.20.10-10.0.20.20"}}
+	if code := e.call(e.token, "POST", "/subnets", in, &mr); code != 201 {
+		t.Fatalf("create: %d %+v", code, mr)
+	}
+	sn := e.eng.State().Subnets[0]
+	// The pools keep the order given.
+	if sn.PoolOrder != "random" || sn.AddressOrder != "reverse" || sn.Pools[0].String() != "10.0.20.100-10.0.20.110:random" || sn.Pools[1].Order != "" {
+		t.Errorf("subnet %+v", sn)
+	}
+	// "listed" and "default" are the defaults, stored as "".
+	if code := e.call(e.token, "PATCH", "/subnets/"+itoa(sn.ID), api.SubnetInput{PoolOrder: ptr("listed"), AddressOrder: ptr("default")}, nil); code != 200 {
+		t.Errorf("patch: %d", code)
+	}
+	if sn = e.eng.State().Subnets[0]; sn.PoolOrder != "" || sn.AddressOrder != "" {
+		t.Errorf("defaults %+v", sn)
+	}
+	var eb api.ErrorBody
+	if code := e.call(e.token, "PATCH", "/settings", api.SettingsInput{AddressOrder: ptr("lowest")}, &eb); code != 422 || eb.Error.Problems[0].Code != "address_order" {
+		t.Errorf("bad order: %d %+v", code, eb)
+	}
+	if code := e.call(e.token, "PATCH", "/subnets/"+itoa(sn.ID), api.SubnetInput{Pools: &[]string{"10.0.20.10-10.0.20.20:lowest"}}, nil); code != 400 {
+		t.Errorf("bad pool order: %d", code)
+	}
+}