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

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