Ashrae Zones Endpoint

Best-effort ASHRAE 169-2021 zone tagging for a page of stations.

Deliberately a separate endpoint from nearby_stations_endpoint, not an inline tag like Köppen: unlike koppen_at_points() (one ~60ms local raster read), ashrae_zone_at_points() makes real NASA POWER network calls (climatology has no batch-point API) plus a per-grid-cell nearest-station snap against an 11.5k-row table -- grid-deduplicated and parallelized, but still ~2-5s for a page of 80 stations. Folding that into the map's initial station search would make every map click feel slow; instead the frontend renders markers immediately from nearby_stations_endpoint and calls this endpoint after, filling in zones as they arrive. Any failure degrades to all-None, same best-effort contract as the Köppen tag.