Normalize event location countries (locations.country) to the country dropdown #2832

🤖 From Claude:

Event venues (locations) still hold free-text countries, so they can disagree with the country dropdown everything else now uses.

Context

• Country dropdown on forms and admin address pages #2805 made every country field a dropdown (GeographyHelper::COUNTRIES).
• Shared address translation for imports and a one-time cleanup #2829 adds AddressTranslation and data:translate_addresses, which bring existing addresses.country and country/state form answers onto the dropdown values (ADR-0009).
• locations.country is the one remaining typed-in country column. It's left out of that cleanup for now.
  • Ahoy's ahoy_visits.country is filled automatically from visitors' IP addresses and is out of scope.

What's needed

• Decide whether Location is still used. The model is marked # TODO remove this class if unused, and nothing in the app creates rows; events pick one in EventsController (@locations).
• If it stays: translate locations.country (and state) through AddressTranslation#country / #state — e.g. a locations pass in AddressServices::TranslateExisting, logged the same way.
• If the registrations importer work touches event locations, route any country/state it writes through AddressTranslation so nothing off-dropdown lands.

Registrations importer rework — build on #2829

• Shared address translation for imports and a one-time cleanup #2829 already routes EventRegistrationImporter through AddressTranslation:
  • translate_address_answers translates the mailing/organization state and country answers.
  • contact_type reads address and phone types the same way.
• An unrecognized country is left blank. The preview lists what the dropdowns don't offer, and the completion notice counts it (Result#untranslated_addresses).
• Keep those calls (or their equivalent) in the reworked importer, and rebase onto Shared address translation for imports and a one-time cleanup #2829 once it merges, so imported registrations only ever store dropdown countries.

Assigned to Justin because he's reworking the registrations importer, which this sits next to.