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).
Maidenhead grid module converts grid squares to lat/lon coordinates.
Submission form validates grids, bands, modes, and email, computes
positions and distance, then triggers the weather/HRRR/terrain
processing pipeline via Oban.