A self-hosted Open-Meteo instance that isn't backfilled yet answers historical
requests with all-null values (or only its recent sync window), which
_finalize_frame reduces to a near-empty frame. That frame is not None, so the
loader would accept it as source=open-meteo and cache it indefinitely as a
complete record — never falling back to NASA and never self-healing after the
backfill lands. Require at least MIN_ARCHIVE_DAYS before accepting an archive
result; below that, fall through to the NASA backup and don't cache the short
frame. Guards the prod cutover window.
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