raise test coverage to 100% (152 tests)

serve pairs from per-city index (numbeo rankings): 361 cities, up to 10/country, all 92 capitals; myanmar->yangon

serve cross-country comparisons from an offline numbeo cache

switch numbeo scraper to curl_cffi with chrome tls impersonation

harden numbeo fetch with browser headers, cookie priming and retry

add test suite covering model, fx, currencies, scraping and endpoints