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

Every cache of answers from an outside service has its own file in `SWW_CACHE_DIR` (default `/var/cache/sww`): `lookups.json` (GeoJS), `lists.json` (lists fetched by URL, CrowdSec's decisions included), `dnsbl.json`, `abuseipdb-scores.json` and `routewatch.json` (at most 100,000 answers, each kept an hour). `reputation.json` is gone; the AbuseIPDB checks spent today stay in `SWW_STATE_DIR` as `abuseipdb-checks-spent.json`, since losing them would spend the daily budget twice.

A cache file never stops the start: one that does not parse is set aside as `<name>.bad` with a `file_error` alert. An unwritable `SWW_CACHE_DIR` stops the start. The `run` script creates the cache directory and gives it to `sww`. The cache directory needs no volume and can be left out of backups.

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