Airports within a radius of a coordinate, sorted by distance. Query: lat (-90..90), lon (-180..180), radius_km (1-2000, default 200), limit (1-100, default 20), type (optional: large_airport|medium_airport|small_airport|heliport|seaplane_base|balloonport|closed), country (optional 2-letter ISO 3166-1), scheduled_service (optional bool, true = commercial-service airports only). Returns { query, count, airports: [{ id, ident, name, iataCode, icaoCode, type, latitude, longitude, distanceKm, ... }] }.