scripts/backfill_hii_waterlevel.py walks the api-v3 waterlevel_graph
endpoint (hourly wl_msl + discharge, archive back to ~2019) in full-year
windows per station and upserts into hii_waterlevel. Defaults to the
RID-mirror and key stations; --stations/--all/--start/--end/--chunk-days
override. History upserts touch only wl_msl and discharge so colliding
live-snapshot rows keep storage_percent/situation_level. Idempotent and
safe to re-run.
