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/proxy/lookup_test.go

@@ -349,6 +349,7 @@ const unansweredGeoJSURL = "unanswered://geojs/v1/ip/geo.json"
 func TestMain(m *testing.M) {
 	transport, _ := http.DefaultTransport.(*http.Transport)
 	transport.RegisterProtocol("unanswered", unansweredGeoJS{})
+	transport.RegisterProtocol("abuseipdb", abuseIPDBStandIn{})
 
 	m.Run()
 }