Create/update a report's steps + entries (legacy PUT /reports/steps)

Requires permission: ReportAdmins

Upserts each step and its mark-grade fields/options, and (when CreateEntries=true) expands every step's entry groups (sets/houses/tutor-groups/individual) to per-pupil entries with seeded mark grades via the canonical enrolment bridge. Port of the legacy v2 PutCreateReportSteps endpoint.