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>

docs/api.md

@@ -71,9 +71,13 @@ Creating answers 201, other changes 200.
 
 ```json
 {"lease_time": 43200, "ping_check": true, "ping_timeout_ms": 500, "decline_hold": 86400,
- "not_authoritative": false, "options": [{"option": "domain-name-servers", "value": "10.0.10.2"}]}
+ "not_authoritative": false, "address_order": "sequential",
+ "options": [{"option": "domain-name-servers", "value": "10.0.10.2"}]}
 ```
 
+`address_order` is `sequential` (left out when it is), `reverse` or
+`random` ([subnets.md](subnets.md#address-order)).
+
 `options` replaces the list. Each option is `{"option": NAME or NUMBER,
 "value": TEXT}`, with `"type"` for numbers the catalog doesn't know
 ([options.md](options.md)).
@@ -90,12 +94,17 @@ Creating answers 201, other changes 200.
 
 ```json
 {"name": "office", "interface": "eth0", "prefix": "10.0.10.0/24",
- "pools": ["10.0.10.100-10.0.10.199", "10.0.10.224/28"], "lease_time": 0,
+ "pools": ["10.0.10.100-10.0.10.199", "10.0.10.224/28:random"], "pool_order": "listed",
+ "address_order": "default", "lease_time": 0,
  "options": [{"option": "routers", "value": "10.0.10.1"}], "disabled": false, "comment": ""}
 ```
 
-`pools` and `options` replace the lists; `lease_time` 0 means the
-settings'.
+`pools` and `options` replace the lists; the pools keep their order. A
+pool's `:ORDER` is its own address order. `pool_order` is `listed` or
+`random`; `address_order` is `sequential`, `reverse`, `random` or
+`default` (the settings'). Subnets come back with pools as
+`{"start", "end", "order"}`, and the defaults (`listed`, `default`) left
+out. `lease_time` 0 means the settings'.
 
 ### Reservations