seed_era5.py backfills the curated-city cells with true ERA5 data from the
Earthmover Icechunk ERA5 archive on AWS Open Data (anonymous, keyless), so
high-traffic pages keep ERA5 fidelity now that NASA POWER is the live primary.
It aggregates the hourly ERA5 point series to the daily store schema in polars
(no pandas), derives feels-like via the existing heat-index/wind-chill path, and
writes straight to the history store via climate._write_history_backed, bypassing
the live fetch. ERA5 carries real gusts (i10fg), so seeded cells keep measured
gusts rather than the Meteostat fallback.
The icechunk/xarray/zarr stack is isolated in requirements-seed.txt (not the app
image or CI) and lazy-imported, so the module and its unit-tested hourly->daily
transform load without those deps. The S3 access layer targets a young API and is
verified via `--dry-run` on the seed box, not in tests. Idempotent by default
(skips already-cached cells; --overwrite to reseed).
