Cache Avatars Pending

Cache up to limit pending KOL avatars from upstream CDN to R2.

Returns a small summary: scanned (rows examined), cached (successful uploads), failed (download / upload errors — those rows tick their avatar_cache_attempts counter; after 3 failures they exit the queue), bytes_uploaded (sum of payload sizes across the batch), r2_configured (false when env vars are missing — short-circuits without touching the DB), and error (set when the early-exit fired).