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/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.