Completes the browse/discover set across all four entity types.
Companies have no popularity or date field, so unlike movies/series/
people there's no sort_by choice -- always alphabetical by name.
Filters are company_type (validated against the enum) and country
(a plain ISO 3166-1 code comparison, nothing to validate).
build_company_summary refactored to share row-to-summary logic with
the new list path, same pattern used for movies/series/people.
Co-Authored-By: Claude Sonnet 5 <[email protected]>