Ban the netblock of a client that breaks a rate limit, in memory (closes #18)

A request over a rate limit is refused with SWWAF_BAN_RESPONSE and bans
the client's netblock: an hour at first, three times the last ban when
broken again within a day of its end, permanent past seven days. The
ban ledger in internal/bans is checked after the static lists and
before the lookup, and the requests it refuses are not counted. A ban
resets the client's counters and carries notes holding the request
that broke the limit, as SPEC.md now says. At most SWWAF_MAX_BANS are
held. SWWAF_BAN_RESPONSE also answers SWWAF_DENY_NETS and the country
lists.

Judgement call: the six ban settings cannot be off.
Judgement call: a permanent ban's ban_expires is "permanent".

Model: opus-5-5

SPEC.md

@@ -948,9 +948,9 @@ and the running `smallwebwaf` takes the edit in.
     - what was broken: the rule ids and target that matched, or the limit, its
       window, the count reached and the client's limit percentage with what set
       it; and any reputation sources that listed the client;
-    - the requests that caused the ban, up to the last ten: time, method, host,
-      path with its query string, status and user agent, each text cut to 256
-      bytes;
+    - the request that caused the ban, the one that broke the limit or carried
+      the clear sign of attack: time, method, host, path with its query string,
+      status and user agent, each text cut to 256 bytes;
     - how many requests counted toward the ban, and the time span over which
       they came;
     - the netblock's total requests since it was first seen, and the requests
@@ -961,13 +961,13 @@ and the running `smallwebwaf` takes the edit in.
       the table is full, so on a public service the file grows to the default
       `SWWAF_MAX_TRACKED_CLIENTS` of 20,000, about 20 MiB. Written every 15
       minutes, that is under 2 GiB of disk writes a day.
-    - `bans.json` takes about 2 KiB per ban and at most about 8 KiB, since the
-      texts in the notes are cut short. At the default `SWWAF_MAX_BANS` of 5,000
-      it is about 10 MiB, and never more than about 40 MiB, plus whatever bans
-      an admin made. It is written when a ban is made, lifted or made permanent,
-      at most once every 10 seconds, and otherwise with the 15-minute write, so
-      its writes follow the bans made: with a full file, a hundred new bans a
-      day come to about 1 GiB of disk writes.
+    - `bans.json` takes about 1.2 KiB per ban and at most about 2.5 KiB, since
+      the notes hold one request and their texts are cut short. At the default
+      `SWWAF_MAX_BANS` of 5,000 it is about 6 MiB, and never more than about 12
+      MiB, plus whatever bans an admin made. It is written when a ban is made,
+      lifted or made permanent, at most once every 10 seconds, and otherwise
+      with the 15-minute write, so its writes follow the bans made: with a full
+      file, a hundred new bans a day come to about 600 MiB of disk writes.
     - `lookups.json` takes about 150 bytes per answer, about 15 MiB when full.
       Written every 15 minutes, that is under 1.5 GiB of disk writes a day.
     - `reputation.json` and `alerts.json` are usually a few MiB or less.