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/h_config.go

@@ -118,6 +118,9 @@ func (s *Server) updateSettings(w http.ResponseWriter, r *http.Request, p *auth.
 		if in.NotAuthoritative != nil {
 			set.NotAuthoritative = *in.NotAuthoritative
 		}
+		if in.AddressOrder != nil {
+			set.AddressOrder = orderInput(*in.AddressOrder)
+		}
 		return nil
 	}, func(st *model.State) any { return st.Settings })
 }
@@ -194,6 +197,16 @@ func (s *Server) getSubnet(w http.ResponseWriter, r *http.Request, p *auth.Princ
 	return nil
 }
 
+// orderInput reads an address or pool order: "default" is "" (inherited,
+// or the default); validation checks the rest.
+func orderInput(s string) string {
+	s = strings.ToLower(strings.TrimSpace(s))
+	if s == "default" {
+		return ""
+	}
+	return s
+}
+
 // applySubnetInput sets the given fields.
 func applySubnetInput(sn *model.Subnet, in *api.SubnetInput) error {
 	if in.Name != nil {
@@ -219,6 +232,12 @@ func applySubnetInput(sn *model.Subnet, in *api.SubnetInput) error {
 			sn.Pools = append(sn.Pools, pool)
 		}
 	}
+	if in.PoolOrder != nil {
+		sn.PoolOrder = orderInput(*in.PoolOrder)
+	}
+	if in.AddressOrder != nil {
+		sn.AddressOrder = orderInput(*in.AddressOrder)
+	}
 	if in.LeaseTime != nil {
 		sn.LeaseTime = *in.LeaseTime
 	}