Record download size on versions

`size` field in bytes should be added to versions, https://digest.ecosyste.ms can calculate it if not provided from the registry.

Start by locating where version metadata is represented and where registry data is populated. Check https://digest.ecosyste.ms for the size calculation when the registry does not provide it. Done means versions expose a byte-count size consistently, whether sourced from the registry or calculated by the digest service.

Written by the indexing model from the issue text.