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/validate/validate.go

@@ -84,6 +84,9 @@ func (c *checker) settings(s model.Settings) {
 	if s.DeclineHold != 0 && (s.DeclineHold < 60 || s.DeclineHold > 30*24*3600) {
 		c.add(Error, "decline_hold_range", "settings", "the decline hold is %d seconds; use 1 minute to 30 days", s.DeclineHold)
 	}
+	if !model.ValidAddressOrder(s.AddressOrder) {
+		c.add(Error, "address_order", "settings", "the address order %q is not one of %s", s.AddressOrder, strings.Join(model.AddressOrders, ", "))
+	}
 	c.options(s.Options, "settings")
 }
 
@@ -173,22 +176,31 @@ func (c *checker) pools(s *model.Subnet, obj string) {
 	if len(s.Pools) == 0 && !s.Disabled {
 		c.add(Warning, "subnet_no_pools", obj, "the subnet has no pools: only clients with a reservation get an address")
 	}
+	if s.PoolOrder != "" && s.PoolOrder != model.PoolsListed && s.PoolOrder != model.PoolsRandom {
+		c.add(Error, "pool_order", obj, "the pool order %q is not listed or random", s.PoolOrder)
+	}
+	if !model.ValidAddressOrder(s.AddressOrder) {
+		c.add(Error, "address_order", obj, "the address order %q is not one of %s", s.AddressOrder, strings.Join(model.AddressOrders, ", "))
+	}
 	network, broadcast := s.Prefix.Addr(), model.Last(s.Prefix)
 	for i, p := range s.Pools {
+		if !model.ValidAddressOrder(p.Order) {
+			c.add(Error, "address_order", obj, "pool %s: the address order %q is not one of %s", p.Range(), p.Order, strings.Join(model.AddressOrders, ", "))
+		}
 		if !p.Start.Is4() || !p.End.Is4() || p.End.Less(p.Start) {
-			c.add(Error, "pool_invalid", obj, "pool %s is not a range of IPv4 addresses", p)
+			c.add(Error, "pool_invalid", obj, "pool %s is not a range of IPv4 addresses", p.Range())
 			continue
 		}
 		if !s.Prefix.Contains(p.Start) || !s.Prefix.Contains(p.End) {
-			c.add(Error, "pool_outside_subnet", obj, "pool %s is not inside %s", p, s.Prefix)
+			c.add(Error, "pool_outside_subnet", obj, "pool %s is not inside %s", p.Range(), s.Prefix)
 			continue
 		}
 		if p.Contains(network) || p.Contains(broadcast) {
-			c.add(Error, "pool_network_address", obj, "pool %s includes the network address %s or the broadcast address %s; start it at %s and end it at %s at most", p, network, broadcast, network.Next(), broadcast.Prev())
+			c.add(Error, "pool_network_address", obj, "pool %s includes the network address %s or the broadcast address %s; start it at %s and end it at %s at most", p.Range(), network, broadcast, network.Next(), broadcast.Prev())
 		}
 		for _, o := range s.Pools[:i] {
 			if o.Start.Is4() && o.End.Is4() && !(p.End.Less(o.Start) || o.End.Less(p.Start)) {
-				c.add(Error, "pool_overlap", obj, "pools %s and %s overlap", o, p)
+				c.add(Error, "pool_overlap", obj, "pools %s and %s overlap", o.Range(), p.Range())
 			}
 		}
 	}
@@ -221,7 +233,7 @@ func (c *checker) routers(s *model.Subnet, global []model.OptionValue, obj strin
 		}
 		for _, p := range s.Pools {
 			if p.Contains(a) {
-				c.add(Warning, "router_in_pool", obj, "the router %s is inside pool %s; it is never handed out while it answers pings, but take it out of the pool", a, p)
+				c.add(Warning, "router_in_pool", obj, "the router %s is inside pool %s; it is never handed out while it answers pings, but take it out of the pool", a, p.Range())
 			}
 		}
 	}
@@ -242,7 +254,7 @@ func (c *checker) hostInterface(s *model.Subnet, env *Env, obj string) {
 		if s.Prefix.Contains(a.Addr()) {
 			for _, p := range s.Pools {
 				if p.Contains(a.Addr()) {
-					c.add(Warning, "pool_includes_server", obj, "pool %s includes this server's own address %s; it is never handed out, but take it out of the pool", p, a.Addr())
+					c.add(Warning, "pool_includes_server", obj, "pool %s includes this server's own address %s; it is never handed out, but take it out of the pool", p.Range(), a.Addr())
 				}
 			}
 			return