When ASOS/RAOB/IEMRE APIs return empty data, store a stub record so
dedup checks (has_surface_observations?, has_sounding?, has_iemre_observation?)
see coverage and don't recreate the same jobs on future backfill runs.
- Add precip_1h_in and wx_codes fields to ASOS surface observations
- Add IEMRE reanalysis schema for radar-derived hourly precipitation
- Add IemreFetchWorker with exponential backoff and idempotency
- Integrate IEMRE enqueue into cron weather backfill pipeline
- All existing QSOs marked iemre_queued=false for automatic backfill
