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: 20000, passed by a client that keeps up 6 a second;
- per day: 300000, passed by a scraper 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

SPEC.md

@@ -369,22 +369,38 @@ Conventions:
   owner's own networks. `SWWAF_RATE_LIMIT_EXEMPT_NETS` bypasses the request and
   byte limits only; the error burst, attack detection and bans still apply.
 - Many people can share one address, as in an office, on a mobile carrier or
-  through Tor. They are one client, and share its limits and its bans. The
-  default limits are sized for about 40 busy people on one address. A known
-  shared address busier than that belongs in `SWWAF_ALLOW_NETS` or
+  through Tor. They are one client, and share its limits and its bans. A known
+  shared address busier than the limits allow belongs in `SWWAF_ALLOW_NETS` or
   `SWWAF_RATE_LIMIT_EXEMPT_NETS`.
 
 ### Request rate limits, per client
 
-Breaking one bans the client, so the defaults sit about twice above what 40
-people busy with heavy web apps make together on one address. Each such person
-makes about 75 requests in a busy minute, and 1,250 an hour for ten hours a day.
+Breaking one bans the client. The defaults suit programmatic use of an API, such
+as an automation fleet on another host, and still stop abusive bots. A fleet of
+20 agents, each making a request every 10 seconds at its busiest, makes 120
+requests a minute and 7,200 an hour. Working around the clock, it makes 172,800
+a day. It stays under all three limits.
 
-A client slower than those 40 people cannot be told from them by counting. Only
-the other checks can catch an abuser that slow. They include the trap paths, the
-rule files, the Core Rule Set and the blocklists. An abuser that trips none of
-them is not caught. A machine that talks to the app all day can still pass the
-day limit. Its address belongs in `SWWAF_RATE_LIMIT_EXEMPT_NETS`.
+- Per minute: 500, about 8 requests a second. It leaves room for the fleet's 120
+  a minute and a few quick first visits with an empty cache. A gitea page opened
+  with an empty cache loads about 40 files. A client firing requests as fast as
+  it can passes the limit within seconds.
+- Per hour: 20,000, about 5.5 requests a second for a whole hour. A client that
+  keeps up 6 a second makes 21,600 in an hour, and passes it, at 360 a minute,
+  under the minute limit. The fleet's 7,200 an hour is about a third of it.
+- Per day: 300,000, about 3.5 requests a second around the clock. A scraper that
+  keeps up 4 a second makes 345,600 a day, and passes it. At 240 a minute and
+  14,400 an hour, it stays under the other two limits.
+
+Counting cannot catch a client that stays under all three limits, such as one
+that keeps up 3 requests a second. Only the other checks can catch an abuser
+that slow. They include the trap paths, the rule files, the Core Rule Set and
+the blocklists. An abuser that trips none of them is not caught. A machine that
+keeps up more than 500 requests a minute passes the minute limit. One that keeps
+up more than about 5.5 a second for an hour passes the hour limit. One that
+keeps up more than about 3.5 a second all day passes the day limit, as two busy
+Actions runners on one address do. Such a machine's address belongs in
+`SWWAF_RATE_LIMIT_EXEMPT_NETS`.
 
 `SWWAF_RATE_LIMIT_EXEMPT_PATHS` takes path prefixes out of the count, such as
 static assets or health checks; their bytes are not counted either. A prefix is
@@ -397,12 +413,12 @@ matched the same way.
 
 ### Byte limits, per client
 
-Each of the same 40 people moves about 256 MiB an hour, for ten hours a day. The
-hour and day limits sit about twice above what they move together. A response's
-bytes are counted when it ends (see step 15). So the minute limit holds two of
-the largest responses, with their requests, ending in the same minute. No single
-download then breaks a limit. An operator who raises `SWWAF_RESPONSE_MAX_BYTES`
-raises the byte limits with it.
+The hour and day limits sit about twice above what 40 people busy with heavy web
+apps move together on one address. Each moves about 256 MiB an hour, for ten
+hours a day. A response's bytes are counted when it ends (see step 15). So the
+minute limit holds two of the largest responses, with their requests, ending in
+the same minute. No single download then breaks a limit. An operator who raises
+`SWWAF_RESPONSE_MAX_BYTES` raises the byte limits with it.
 
 ### Size and time limits