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/api/types.go

@@ -68,19 +68,25 @@ type SettingsInput struct {
 	PingTimeout      *int           `json:"ping_timeout_ms,omitempty"`
 	DeclineHold      *int           `json:"decline_hold,omitempty"`
 	NotAuthoritative *bool          `json:"not_authoritative,omitempty"`
+	// AddressOrder is sequential, reverse or random.
+	AddressOrder *string `json:"address_order,omitempty"`
 }
 
 // SubnetInput creates or changes a subnet (unset fields stay). Pools are
-// "FIRST-LAST" or prefixes.
+// "FIRST-LAST" or prefixes, with ":ORDER" for a pool's own address order.
 type SubnetInput struct {
-	Name      *string        `json:"name,omitempty"`
-	Interface *string        `json:"interface,omitempty"`
-	Prefix    *string        `json:"prefix,omitempty"`
-	Pools     *[]string      `json:"pools,omitempty"`
-	LeaseTime *int           `json:"lease_time,omitempty"`
-	Options   *[]OptionInput `json:"options,omitempty"`
-	Disabled  *bool          `json:"disabled,omitempty"`
-	Comment   *string        `json:"comment,omitempty"`
+	Name      *string   `json:"name,omitempty"`
+	Interface *string   `json:"interface,omitempty"`
+	Prefix    *string   `json:"prefix,omitempty"`
+	Pools     *[]string `json:"pools,omitempty"`
+	// PoolOrder is listed or random; AddressOrder sequential, reverse,
+	// random or "" (the settings').
+	PoolOrder    *string        `json:"pool_order,omitempty"`
+	AddressOrder *string        `json:"address_order,omitempty"`
+	LeaseTime    *int           `json:"lease_time,omitempty"`
+	Options      *[]OptionInput `json:"options,omitempty"`
+	Disabled     *bool          `json:"disabled,omitempty"`
+	Comment      *string        `json:"comment,omitempty"`
 }
 
 // ReservationInput creates or changes a reservation (unset fields stay).