Delete a report step + its entries (legacy DELETE /reports/steps)

Requires permission: ReportAdmins

Removes the step together with everything under it: its pupil entries, their mark grades and pre-AI snapshots, and the step's own mark-grade fields and options (the fields and options were previously left behind, so a deleted step's marks and grades outlived it). Port of the legacy v2 DeleteStep endpoint. Exposed at both /reports/step and /reports/steps (the Flutter setup editor and step viewer call the plural and singular forms respectively). Deleting a step that is already gone is not an error.