Refactor project structure: replace User class with ChatUser, integrate Database class, and update CMake configuration for new files

database.h

@@ -0,0 +1,23 @@
+#ifndef YC_LIB_DATABASE_H
+#define YC_LIB_DATABASE_H
+
+#include <memory>
+#include <pqxx/pqxx>
+#include "config.h"
+
+namespace Yc {
+namespace Lib {
+
+class Database {
+public:
+    Database(std::shared_ptr<Config> config);
+    ~Database() = default;
+    pqxx::result exec(const std::string& query);
+private:
+    std::unique_ptr<pqxx::connection> _connection;
+};
+
+} // namespace Lib
+} // namespace Yc
+
+#endif // YC_LIB_DATABASE_H