New service with S3 upload functions for the refractor card art
pipeline. backfill_variant_image_url reads rendered PNGs from disk,
uploads to S3, and sets image_url on BattingCard/PitchingCard rows.
18 tests covering key construction, URL formatting, upload params,
and error swallowing.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
