check-ip now takes only --serve, --geoip-conf, --blocklist-repo,
--cache-dir. Blocklist always comes from git; GeoIP mmdbs always go
through httpcache (when GeoIP.conf is available). Format negotiation
lives entirely server-side.
main.go is now straight-line wiring: parse flags, build the two
databases, run the server. All filesystem setup (MkdirAll for clone
target, for cache Path parents) is pushed into gitshallow and
httpcache so the cmd doesn't do filesystem bookkeeping.
main.go now reads top-to-bottom as setup + usage of the three
databases (blocklists group, whitelist cohort, geoip readers), then
dispatch to one-shot or serve. HTTP server code moved to server.go.
No behavior change.
