Replaces the one-shot startup Task with a GenServer that checks every
5 minutes whether propagation scores are older than 2 hours. If stale,
enqueues a PropagationGridWorker job (with dedup check to avoid double
queuing). Disabled in test env to avoid SQL Sandbox conflicts.
On app start, check if latest scores are older than 2 hours. If so,
immediately enqueue a PropagationGridWorker job. Covers app restarts,
deploys, and missed cron ticks.
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.
