Get Student Evaluation

Minimal: drops exercise_id (lives in the URL) and exercise_collection / created_at (frontend doesn't read them today). author is flattened to a Literal to avoid leaking prompt ids / user ids.

scoring_scheme is appended last and is derived, never stored — a new bundle reads it to decide which shape to render; an old one ignores it.

What was held back from this correction (EXC-861). Only the levels travel: the original text of a reworded or hidden item never leaves the server, the rubric and evaluation beside this already carry the student-facing wording in its place.