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/model/config_test.go

@@ -0,0 +1,22 @@
+package model
+
+import "testing"
+
+func TestParsePoolOrder(t *testing.T) {
+	for in, want := range map[string]string{
+		"10.0.0.1-10.0.0.50":          "10.0.0.1-10.0.0.50",
+		"10.0.0.1 - 10.0.0.50:Random": "10.0.0.1-10.0.0.50:random",
+		"10.0.0.0/26:reverse":         "10.0.0.0-10.0.0.63:reverse",
+		"10.0.0.1-10.0.0.50:default":  "10.0.0.1-10.0.0.50",
+	} {
+		p, err := ParsePool(in)
+		if err != nil || p.String() != want {
+			t.Errorf("%q: %v %v", in, p, err)
+		}
+	}
+	for _, in := range []string{"10.0.0.1-10.0.0.50:", "10.0.0.1-10.0.0.50:lowest"} {
+		if _, err := ParsePool(in); err == nil {
+			t.Errorf("%q accepted", in)
+		}
+	}
+}