Get Class Appreciation

Class-wide narrative appreciation. Auto-generated on first visit AFTER the assignment's due date passes; cached against the class-evals ETag so subsequent visits short-circuit. class_id narrows it to one class of the devoir, gated on that class's own due date (EXC-909); without it, every class the teacher has on the devoir. Returns 404 with an empty body when the assignment is unknown, the class is not one of the teacher's on it, it is not yet overdue, or there are no sessions to summarize — the frontend treats them all identically (empty state). Mirrors the no-annotation return type of get_class_evaluations above so the 304/404 short-circuits don't fight pyright.