Track GeoLite2-ASN.mmdb in git; fail build if absent

Remove docker/geoip/.gitignore so the .mmdb is versioned alongside the
codebase. The Dockerfile now exits non-zero if the file is missing,
making the build self-validating.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/geoip/.gitignore

@@ -1,3 +0,0 @@
-# GeoLite2 binary databases are git-ignored (large binaries, updated frequently).
-# Run update_geoip.sh before each docker build to refresh.
-*.mmdb