ListFeedDigest returns a pre-aggregated digest of all RSS feeds for a site variant.
Site variant: full, tech, finance, happy
ISO 639-1 language code (en, fr, ar, etc.)
Successful response
Per-category buckets — keys match category names from feed config
Per-feed status — only non-ok states emitted; absent key implies ok. Values: empty (feed returned 0 items), timeout (timed out during fetch).
ISO 8601 timestamp of when this digest was generated