PUT full-replace deletes exam_questions, and mark_entries.question_id cascades
ON DELETE — so re-saving the setup canvas after marking began would silently
wipe recorded marks. Guard: 409 if any mark_entry exists for the template's
batches. Mark-scheme edits (PATCH /questions/{id}) are unaffected.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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