Reorder Exercises

Reorder the exam's structured_exercises to match the given id sequence.

The request must list every existing exercise id exactly once — partial reorderings or unknown ids are rejected so the new version is always a pure permutation of the previous one (EXC-66).

The ids are then reassigned 1..n in the new order, because an exercise's id is its number everywhere it is shown: the card badge, the exported PDF heading and what the chat means by « exercice 2 ». Every other write path already keeps that invariant — additions append max(id) + 1, both deletion paths reindex — and reordering without it left a subject numbered 4, 1, 3, 2 (EXC-835).