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/metrics.md

@@ -58,9 +58,9 @@ By `source`: each list's URL, each zone, and `abuseipdb`:
 - `sww_reputation_last_fetch_timestamp_seconds`, for a list;
 - `sww_reputation_daily_budget_remaining`, for AbuseIPDB.
 
-## State files
+## State and cache files
 
-By `file`:
+By `file`, the name of a state or cache file:
 
 - `sww_state_file_writes_total`, `sww_state_file_write_failures_total`,
   `sww_state_file_last_write_timestamp_seconds` and `sww_state_file_size_bytes`;