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

docs/alerts.md

@@ -23,7 +23,8 @@ back or dropped.
     - the day's AbuseIPDB checks used up.
 - `file_error`: one of:
     - a rule file or state file edit with an error;
-    - a state file that cannot be written;
+    - 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 lookup database that cannot be read.
 - `summary`: the hourly summary of alerts held back.