DNS blocklists asked in the background, verdicts kept (closes #104)

Zones in SWWAF_DNSBL_ZONES are asked about each client in the background,
through SWWAF_DNSBL_RESOLVER or the host's resolver; no request waits.
Verdicts last SWWAF_REPUTATION_CACHE_TTL, kept in reputation.json.
SWWAF_REPUTATION_ACTION (limit:25) denies, limits or logs a listed
client; each zone listing it raises reputation_hit. A failed query gives
no verdict, raises source_failure, and pauses the zone a minute. A
zone's key, its first label under dq.spamhaus.net, is masked everywhere
but reputation.json. Zones compare without regard to case.

Judgement call: answers in 127.255.255.0/24 or outside 127.0.0.0/8 are failures.
Judgement call: the minute's pause after a failure; at most 1,000 queries at once.
Judgement call: one zone given with two keys stops the start as listed twice.
Rule suppressed: paralleltest on the DNSBL tests (Go's resolver shares state across synctest bubbles), funlen on the test of every logged setting.

Model: opus-5-5

internal/alerts/alerts.go

@@ -46,10 +46,11 @@ const (
 	EventAnomaly = "anomaly"
 	// EventWAFBlock comes with the Core Rule Set; nothing raises it yet.
 	EventWAFBlock = "waf_block"
-	// EventReputationHit is a request whose client a blocklist lists.
+	// EventReputationHit is a request whose client a blocklist or a DNSBL
+	// zone lists.
 	EventReputationHit = "reputation_hit"
-	// EventSourceFailure is GeoJS failing or refusing smallwebwaf, or a
-	// fetch of a list failing.
+	// EventSourceFailure is GeoJS failing or refusing smallwebwaf, a fetch
+	// of a list failing, or a query to a DNSBL zone failing or refused.
 	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