Adds haversine_km/4 to Radio context and backfill_distances/1 which
calculates distance_km from pos1/pos2 for QSOs missing it. Called
by QsoWeatherEnqueueWorker before enqueueing weather fetches.
Oban cron worker runs every 4 hours, finds QSOs without weather data,
discovers nearby ASOS/sounding stations, and enqueues WeatherFetchWorker
jobs. Migrations run automatically on app start in production.
