feat: expand platform management, admin controls, and learning workflows

backend/src/domain/json.cc

@@ -40,6 +40,7 @@ Json::Value ToJson(const Submission& s) {
   j["language"] = ToString(s.language);
   j["status"] = ToString(s.status);
   j["score"] = s.score;
+  j["rating_delta"] = s.rating_delta;
   j["time_ms"] = s.time_ms;
   j["memory_kb"] = s.memory_kb;
   j["compile_log"] = s.compile_log;