AbuseIPDB scores for clients that broke a limit, within a daily budget (closes #105)

With SWWAF_ABUSEIPDB_KEY set, a client whose history counts an offence
is checked in the background, at most SWWAF_ABUSEIPDB_DAILY_BUDGET checks
a day, the count kept in reputation.json. A score at or over
SWWAF_ABUSEIPDB_MIN_SCORE is a hit for SWWAF_REPUTATION_ACTION, logged as
abuseipdb and alerted with its score. A failure or the used-up budget
gives no score and raises source_failure. The key goes only in the Key
header.

Judgement call: the budget's day is UTC; AbuseIPDB documents no reset time.
Judgement call: each check sent spends budget; a minute's pause after a failure.
Judgement call: offences are those the history counts, so far broken limits only.

Model: opus-5-5

internal/alerts/alerts.go

@@ -47,10 +47,11 @@ const (
 	// EventWAFBlock comes with the Core Rule Set; nothing raises it yet.
 	EventWAFBlock = "waf_block"
 	// EventReputationHit is a request whose client a blocklist or a DNSBL
-	// zone lists.
+	// zone lists, or whose AbuseIPDB score is a hit.
 	EventReputationHit = "reputation_hit"
 	// EventSourceFailure is GeoJS failing or refusing smallwebwaf, a fetch
-	// of a list failing, or a query to a DNSBL zone failing or refused.
+	// of a list failing, a query to a DNSBL zone or a check with AbuseIPDB
+	// failing or refused, or the day's AbuseIPDB checks used up.
 	EventSourceFailure = "source_failure"
 	// EventFileError is a rule file or state file edited while smallwebwaf
 	// runs that does not parse, a replacement of the lookup database that