The old ParseConf opened the file itself, which the name did not
convey. Now it parses the config text directly, matching
encoding/json.Unmarshal-style conventions: callers read the file (or
source the string however they like) and pass it in. Also introduce
errors.ErrMissingCredentials for the credential-missing case so callers
can branch on it.
- drop Checker struct, loadCohort helper, and contains() nil-wrapper
- inline check logic into server as a closure
- geoip.Databases: no nil-receiver guards, no nil-field branches, no
"disabled" mode. City + ASN are both required; caller hands explicit
paths and OpenDatabases returns a fully-initialized value or an err
- main.go is now straight-line wiring with no helper functions
geoip.ParseConf() extracted from geoip-update into the geoip package so
both cmds can read GeoIP.conf without duplication.
check-ip gains -geoip-conf flag: reads AccountID+LicenseKey, resolves
mmdb paths into data-dir, builds httpcache.Cachers with geoip.NewCacher.
Background runLoop now refreshes both blocklists and GeoIP DBs on each
tick, hot-swapping geoip2.Reader via atomic.Pointer.Swap + old.Close().
