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/kea/kea.go

@@ -90,6 +90,9 @@ type importer struct {
 	opts    Options
 	res     *Result
 	globals []string // interfaces-config interfaces
+	// allocator is Dhcp4's allocator (iterative, random or flq).
+	allocator string
+	noted     map[string]bool
 }
 
 // ParseConfig reads a kea-dhcp4.conf.
@@ -131,6 +134,17 @@ func jsonError(b []byte, err error) error {
 	return err
 }
 
+// noteOnce adds a note once per key.
+func (im *importer) noteOnce(key, format string, args ...any) {
+	if im.noted == nil {
+		im.noted = map[string]bool{}
+	}
+	if !im.noted[key] {
+		im.noted[key] = true
+		im.note(format, args...)
+	}
+}
+
 func (im *importer) note(format string, args ...any) {
 	im.res.Notes = append(im.res.Notes, fmt.Sprintf(format, args...))
 }
@@ -154,7 +168,7 @@ func get[T any](o obj, key string, into *T) (bool, error) {
 // note.
 var (
 	handledKeys = map[string]bool{"interfaces-config": true, "valid-lifetime": true, "option-data": true, "subnet4": true, "shared-networks": true,
-		"reservations": true, "authoritative": true, "decline-probation-period": true}
+		"reservations": true, "authoritative": true, "decline-probation-period": true, "allocator": true}
 	ignoredKeys = map[string]string{
 		"lease-database": "Steward keeps leases in its own database", "control-socket": "Steward has its own API",
 		"loggers": "Steward logs to the journal", "renew-timer": "Steward sends half the lease time",
@@ -195,6 +209,7 @@ func (im *importer) dhcp4(d obj) error {
 			im.globals = append(im.globals, name)
 		}
 	}
+	get(d, "allocator", &im.allocator)
 	set := &im.res.State.Settings
 	var vl int
 	if ok, err := get(d, "valid-lifetime", &vl); err != nil {
@@ -346,6 +361,18 @@ func (im *importer) subnet(s obj, shared obj) error {
 		}
 		sn.Options = so
 	}
+	alloc := im.allocator
+	if shared != nil {
+		get(shared, "allocator", &alloc)
+	}
+	get(s, "allocator", &alloc)
+	switch alloc {
+	case "random":
+		// Kea picks among all the subnet's free addresses.
+		sn.PoolOrder, sn.AddressOrder = model.PoolsRandom, model.OrderRandom
+	case "flq":
+		im.noteOnce("flq", "the allocator flq (free lease queue) became Steward's sequential order: a new client gets the lowest address never handed out, else the one free the longest")
+	}
 	if ok, _ := get(s, "interface", &iface); ok && iface == "" {
 		iface = ""
 	}