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

db_example.cpp

@@ -0,0 +1,17 @@
+#include <iostream>
+#include <pqxx/pqxx>
+
+int main() {
+    try {
+        pqxx::connection c{"dbname=your_db user=your_user password=your_pass host=localhost"};
+        pqxx::work txn{c};
+        pqxx::result r = txn.exec("SELECT version();");
+        for (auto row : r) {
+            std::cout << row[0].c_str() << std::endl;
+        }
+    } catch (const std::exception &e) {
+        std::cerr << "PostgreSQL error: " << e.what() << std::endl;
+        return 1;
+    }
+    return 0;
+}