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/dhcpd/handle.go

@@ -156,29 +156,24 @@ func (s *Server) choose(cfg *config, iface string, subs []*subnet, cl client, re
 			return hold(requested, sub, true)
 		}
 	}
-	// 4. A free address: one never handed out, else the one free the
-	// longest.
+	// 4. A free address: one never handed out, in the subnet's order
+	// (order.go), else the one free the longest.
+	for _, sub := range subs {
+		if ip, ok := s.neverUsed(cfg, own, sub, cl, now); ok {
+			return hold(ip, sub, true)
+		}
+	}
 	var oldest netip.Addr
 	var oldestSub *subnet
 	var oldestEnd time.Time
 	for _, sub := range subs {
 		for _, pool := range sub.Pools {
-			for v := model.U32(pool.Start); v <= model.U32(pool.End) && v != 0; v++ {
-				ip := model.FromU32(v)
-				if !s.usable(cfg, own, sub, ip, cl, now) {
-					continue
-				}
-				l := s.leases[ip]
-				if l == nil {
-					return hold(ip, sub, true)
-				}
-				if !oldest.IsValid() || l.End.Before(oldestEnd) {
+			eachIn(pool, false, func(ip netip.Addr) bool {
+				if l := s.leases[ip]; l != nil && s.usable(cfg, own, sub, ip, cl, now) && (!oldest.IsValid() || l.End.Before(oldestEnd)) {
 					oldest, oldestSub, oldestEnd = ip, sub, l.End
 				}
-				if v == model.U32(pool.End) {
-					break
-				}
-			}
+				return false
+			})
 		}
 	}
 	if oldest.IsValid() {