httpcache.Cacher gains:
- Username/Password: Basic Auth, stripped before following redirects
- MaxAge: skip HTTP if local file mtime is within this duration
- MinInterval: skip HTTP if last Fetch attempt was within this duration
- Transform: post-process response body (e.g. extract .mmdb from tar.gz)
geoip.Downloader now builds an httpcache.Cacher via NewCacher(), removing
its own HTTP client. ExtractMMDB is now exported for use as a Transform.
check-ip-blacklist renamed to check-ip; adds -city-db / -asn-db flags
for GeoLite2 lookup (country, city, subdivision, ASN) printed after each
blocklist result.
Top-layer callers (IPFilter) now drive all reloads directly after
Sync/Fetch return. gitshallow.Init now returns (bool, error).
httpcache drops Init and Sync — callers just call Fetch.
Blacklist → IPFilter with three separate atomic cohorts: whitelist
(never blocked), inbound, and outbound. ContainsInbound/ContainsOutbound
each skip the whitelist. HTTP sync fetches all cachers before a single
reload to avoid double-load. Also fixes httpcache.Init calling c.Fetch().
