Caches of outside APIs live in /var/cache/sww (closes #314)

The answers of GeoJS, the lists fetched by URL, the DNS blocklists,
AbuseIPDB and routewatch each go in a cache file in SWW_CACHE_DIR
(default /var/cache/sww): lookups.json, lists.json, dnsbl.json,
abuseipdb-scores.json and routewatch.json. reputation.json is gone. The
AbuseIPDB checks spent today are a count, not a cache, and stay in
SWW_STATE_DIR as abuseipdb-checks-spent.json.

A cache file never stops the start: one that does not parse is set
aside as an edit is, one that cannot be read is written over, and its
cache starts empty. An unwritable SWW_CACHE_DIR stops the start, as an
unwritable SWW_STATE_DIR does. Each directory is watched on its own.
The run script makes the cache directory and gives it to sww.

Model: opus-5-5

internal/alerts/alerts.go

@@ -56,9 +56,11 @@ const (
 	// 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 sww
-	// runs that does not parse, a replacement of the lookup database that
-	// cannot be read, or a state file that cannot be written.
+	// EventFileError is one of:
+	//   - a rule file or state file edited while sww runs that does not parse;
+	//   - a cache file that does not parse, at start or after an edit;
+	//   - a state file or cache file that cannot be written;
+	//   - a replacement of the lookup database that cannot be read.
 	EventFileError = "file_error"
 	// EventSummary is the summary sent as an hour ends: of the alerts held
 	// back in it past SWW_ALERT_MAX_PER_HOUR, and of the repeats held