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

internal/config/config_test.go

@@ -262,9 +262,9 @@ func TestDefaults(t *testing.T) {
 		UpstreamResponseTimeout:     30 * time.Minute,
 		RequestMaxBytes:             100 << 20,
 		ResponseMaxBytes:            1 << 30,
-		RateLimitPerMinute:          6000,
-		RateLimitPerHour:            100000,
-		RateLimitPerDay:             1000000,
+		RateLimitPerMinute:          500,
+		RateLimitPerHour:            20000,
+		RateLimitPerDay:             300000,
 		BanResponse:                 403,
 		LimitBanDuration:            time.Hour,
 		LimitBanRepeatWindow:        24 * time.Hour,
@@ -2375,9 +2375,9 @@ func TestLogsEachSettingWithItsValue(t *testing.T) {
 		allowNets:               "",
 		rateLimitExemptNets:     "",
 		denyNets:                "",
-		rateLimitPerMinute:      "6000",
-		rateLimitPerHour:        "100000",
-		rateLimitPerDay:         "1000000",
+		rateLimitPerMinute:      "500",
+		rateLimitPerHour:        "20000",
+		rateLimitPerDay:         "300000",
 		rateLimitExemptPaths:    "",
 		bytesLimitPerMinute:     "4G",
 		bytesLimitPerHour:       "20G",