Remove visibility overrides for a group

Remove visibility overrides for the group.

Items identify rows by (published_course_id, is_visible). Each item is processed in its own transaction.atomic() block so a per-row failure does not abort the rest (no outer atomic wrapping; each per-row block opens a fresh transaction). Within-batch dedup is first-wins on the same tuple.

Returns 404 if the group is not in the caller's organization.

The id field on BatchDeletedItem echoes the published_course_id from the request (not the override's own obfuscated id), so clients can correlate inputs to outcomes without keeping a separate lookup table.

The group lookup runs before the envelope guard so a missing group is reported as 404 regardless of body content (consistent with the GET / POST relationship endpoints).

Required OAuth scope: student-groups:write