Default request limits per minute, hour and day, sized for API use (closes #176)

The default request limits now suit programmatic use of an API, such
as an automation fleet on another host, and still catch abusive bots:

- per minute: 500, room for a fleet's busiest minute and a few first
  visits with an empty cache, and passed within seconds by a client
  firing as fast as it can;
- per hour: 30000, which a client at the minute limit for a whole
  hour reaches exactly;
- per day: 300000, passed by a crawler that keeps up 4 a second.

The byte limits and ban lengths are unchanged. SPEC.md, README.md and
docs/settings.md give the reasons; docs/gitea.md redoes the Actions
runner sums.

Judgement call: the docs/alerts.md ban example shows the minute limit.

Model: opus-5-5

docs/settings.md

@@ -99,9 +99,21 @@ that take `off`, the `_FILE` form, and what each group does and why.
 
 ## Request rate limits, per client
 
-- `SWWAF_RATE_LIMIT_PER_MINUTE` (default `6000`), `SWWAF_RATE_LIMIT_PER_HOUR`
-  (default `100000`) and `SWWAF_RATE_LIMIT_PER_DAY` (default `1000000`): the
-  most requests a client may make in each window. A count or `off`.
+- `SWWAF_RATE_LIMIT_PER_MINUTE` (default `500`, about 8 a second): the most
+  requests a client may make in a minute. A count or `off`. The default catches,
+  within seconds, a client firing requests as fast as it can. It leaves room for
+  a fleet of about 20 agents at its busiest, about 120 requests a minute. It
+  also leaves room for a few quick first visits to gitea with an empty cache,
+  about 40 files each.
+- `SWWAF_RATE_LIMIT_PER_HOUR` (default `30000`, about 8 a second): the most
+  requests a client may make in an hour. A count or `off`. The default leaves
+  room for an automation fleet at work. A client at the minute limit for a whole
+  hour reaches it exactly.
+- `SWWAF_RATE_LIMIT_PER_DAY` (default `300000`, about 3.5 a second): the most
+  requests a client may make in a day. A count or `off`. The default catches a
+  scraper that keeps up 4 a second around the clock.
+- Counting cannot catch a client that stays under all three limits (see "Request
+  rate limits, per client" in [`SPEC.md`](../SPEC.md)).
 - `SWWAF_RATE_LIMIT_EXEMPT_PATHS` (default empty): path prefixes exempt from
   rate limiting and the byte limits, such as `/assets/`.