Accept an incoming board value over a hand-changed one

Requires permission: ExamAdmin

An import leaves a hand-changed grade or mark alone and reports the conflict. This applies the incoming value for ONE result, explicitly, and hands the field back to the imports so a later corrected file flows through normally. Recorded as AcceptedImport — never as a manual correction, because the two mean opposite things to the protection rule. The earlier hand edit stays in the history; it simply stops protecting the row.