Add a whole subject set to a report (legacy PUT /reports/reallocation/set)

Requires permission: ReportAdmins

Inserts one entry per pupil in the set (resolved via the canonical Class/ClassStudents bridge) attributed to the chosen teacher, optionally seeding each entry's mark-grades from the step's grade fields. Port of the legacy RPT_RA_MarkGradesForReport + RPT_InsertEntry/RPT_InsertMarkGrade chain.