feat: Refresh subnet countries with bulk lookups #13

Problem

Country is checked only during import with one API request per subnet. Failed lookups remain empty, existing values can become outdated, and many individual requests may trigger rate limiting.

Proposed change

Use the country.is bulk endpoint everywhere country lookup is needed, batching up to 100 subnet sample IPs per request. Fully remove the single-IP country lookup path.

Refresh the country of every probed subnet as part of probing, even when it already has a country, so missing or changed geolocation data is updated and persisted. Imports should use the same bulk lookup implementation.

Alternatives considered

Retrying individual lookups or refreshing only missing countries would not address rate limiting or countries that change over time.

Scope

• This request fits proxybench's single-window proxy subnet workflow.