20 attempts with exponential backoff capped at 6 hours ensures
data is eventually fetched even during extended rate limiting or
API outages (~3 days of retrying before giving up).
IEM was returning 503s under 10 concurrent workers. Three fixes:
- Req transient retry with exponential backoff on IEM requests
- WeatherFetchWorker Oban job retries failed ASOS/RAOB fetches 2-3h later
with random jitter to avoid thundering herd
- Import script concurrency reduced 10→5, weather queue capped at 3
