feat: edit quiz questions over MCP

- quizzes_create_question, update_question, delete_question and
  reorder_questions on /api/v1/quizzes/{slug}/questions
- update_question replaces answers by id (kept, created, deleted), so
  answer ids stored in past attempts stay valid; foreign ids are refused
- classes.CheckQuestion holds the importer's per-question rules, shared
  with the manage routes; attempt scores are never touched (D-16)

plugin_test.go

@@ -77,6 +77,10 @@ func TestRoutesRegistered(t *testing.T) {
 		"POST /api/v1/quizzes/{slug}/publish",
 		"POST /api/v1/quizzes/{slug}/unpublish",
 		"GET /api/v1/quizzes/{slug}/stats",
+		"POST /api/v1/quizzes/{slug}/questions",
+		"PATCH /api/v1/quizzes/{slug}/questions/{id}",
+		"DELETE /api/v1/quizzes/{slug}/questions/{id}",
+		"PUT /api/v1/quizzes/{slug}/questions/order",
 	}
 	var got []string
 	for _, r := range router.Routes() {