Add DBIP city database and improve geo test coverage

- Add dbip-city-lite package to devenv for GeoIP testing
- Set GEOIP_DATABASE_PATH env var to point to the MMDB database
- Add tests for initGeoReader double-init and error handling
- Add real database tests for IP lookups (US, AU, DE, GB)
- Make real database tests conditional with describe.skipIf
- Improve test coverage from ~97% to 98.82%

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

devenv.nix

@@ -7,6 +7,7 @@
     git
     dbmate
     ast-grep
+    dbip-city-lite
   ];
 
   dotenv.enable = true;
@@ -39,6 +40,7 @@
   env = {
     DATABASE_URL = "postgres://reviq:reviq@localhost/reviq-dashboard?sslmode=disable";
     TEST_DATABASE_URL = "postgres://reviq:reviq@localhost/reviq-dashboard_test?sslmode=disable";
+    GEOIP_DATABASE_PATH = "${pkgs.dbip-city-lite}/share/dbip/dbip-city-lite.mmdb";
   };
 
   scripts = {