Require completed transfer fields

[chrishoage]

Summary

Correct the /core/transferred response schema to reflect the fields rclone provides for completed transfers.

This endpoint returns completed transfers only. Each completed transfer snapshot includes its identity, progress, timestamps, operation type, group, and error value, but the current schema marks most of these fields as optional.

Changes

• Require group, name, size, bytes, checked, and what
• Require started_at, completed_at, and error
• Keep srcFs and dstFs optional
• Regenerate openapi.json and types.d.ts