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/settings.md

@@ -50,14 +50,16 @@ the two (see "The image's command" below).
 - `SWW_METRICS_TOKEN` (default unset): the token for `/_sww/metrics`. At least
   32 characters.
 
-## State files
+## State and cache files
 
 - `SWW_STATE_DIR` (default `/var/lib/sww`): the directory of the state files, an
   absolute path that exists and can be written.
 - `SWW_STATE_WRITE_DELAY` (default `10s`): how long `bans.json` waits to be
   written after a ban is made, lifted or made permanent. A duration.
-- `SWW_STATE_COUNTER_INTERVAL` (default `15m`): how often every state file is
-  written. A duration.
+- `SWW_STATE_COUNTER_INTERVAL` (default `15m`): how often every state and cache
+  file is written. A duration.
+- `SWW_CACHE_DIR` (default `/var/cache/sww`): the directory of the cache files,
+  an absolute path that can be written. The image's `run` script makes it.
 
 ## Logging