Lays the groundwork for a Day 2-7 propagation outlook driven by NCEP
GEFS ensemble-mean output, picking up where HRRR's 18-hour horizon
leaves off.
- GefsClient: NOMADS URL builder, forecast-hour list (3h to +240,
6h to +384), ensemble-mean message inventory, and a Magnus
dewpoint derivation (pgrb2a publishes RH rather than Td)
- gefs_profiles table + GefsProfile schema mirroring hrrr_profiles
so the scorer can run over rows from either source
- Weather.upsert_gefs_profile/1 and upsert_gefs_profiles_batch/1
No worker or UI yet — those land in follow-up commits.
