feat(domain): add entities and json helpers for sqlite schema

backend/src/domain/json.cc

@@ -0,0 +1,54 @@
+#include "csp/domain/json.h"
+
+namespace csp::domain {
+
+Json::Value ToPublicJson(const User& u) {
+  Json::Value j;
+  j["id"] = Json::Int64(u.id);
+  j["username"] = u.username;
+  j["rating"] = u.rating;
+  j["created_at"] = Json::Int64(u.created_at);
+  return j;
+}
+
+Json::Value ToJson(const Problem& p) {
+  Json::Value j;
+  j["id"] = Json::Int64(p.id);
+  j["slug"] = p.slug;
+  j["title"] = p.title;
+  j["statement_md"] = p.statement_md;
+  j["difficulty"] = p.difficulty;
+  j["source"] = p.source;
+  j["created_at"] = Json::Int64(p.created_at);
+  return j;
+}
+
+Json::Value ToJson(const Submission& s) {
+  Json::Value j;
+  j["id"] = Json::Int64(s.id);
+  j["user_id"] = Json::Int64(s.user_id);
+  j["problem_id"] = Json::Int64(s.problem_id);
+  j["language"] = ToString(s.language);
+  j["status"] = ToString(s.status);
+  j["score"] = s.score;
+  j["time_ms"] = s.time_ms;
+  j["memory_kb"] = s.memory_kb;
+  j["created_at"] = Json::Int64(s.created_at);
+  return j;
+}
+
+Json::Value ToJson(const WrongBookItem& w) {
+  Json::Value j;
+  j["user_id"] = Json::Int64(w.user_id);
+  j["problem_id"] = Json::Int64(w.problem_id);
+  if (w.last_submission_id.has_value()) {
+    j["last_submission_id"] = Json::Int64(*w.last_submission_id);
+  } else {
+    j["last_submission_id"] = Json::nullValue;
+  }
+  j["note"] = w.note;
+  j["updated_at"] = Json::Int64(w.updated_at);
+  return j;
+}
+
+}  // namespace csp::domain