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. Returns 404 with an empty body when the assignment is unknown, not yet overdue, or has no sessions to summarize — the frontend treats all three identically (empty state). Mirrors the no-annotation return type of get_class_evaluations above so the 304/404 short-circuits don't fight pyright.