feat: DB 커넥션 풀 기능 추가

TODO: 동시에 여러 DB 연결 확인해야함. 현재 기능만 확인 완료

include/DatabaseManager.h

@@ -39,18 +39,38 @@ public:
   boost::asio::awaitable<RankingResult> GetRanking(int type,
                                                    std::string nickname);
 
+  //  내부용 객체
+  class PooledConnection {
+  public:
+    PooledConnection(DatabaseManager &manager,
+                     std::unique_ptr<boost::mysql::tcp_ssl_connection> conn);
+    ~PooledConnection();
+
+    boost::mysql::tcp_ssl_connection &Get();
+    boost::mysql::tcp_ssl_connection *operator->();
+
+  private:
+    DatabaseManager &manager_;
+    std::unique_ptr<boost::mysql::tcp_ssl_connection> conn_;
+  };
+
+  boost::asio::awaitable<std::unique_ptr<PooledConnection>> GetConnection();
+
 private:
   DatabaseManager() = default;
+  void ReturnConnection(std::unique_ptr<boost::mysql::tcp_ssl_connection> conn);
 
-  // 비동기 Mutex 역할을 할 함수들
-  boost::asio::awaitable<void> Lock();
-  void Unlock();
+  // DB 접속 정보 저장용 (새로운 커넥션 생성 시 필요)
+  std::string host_, user_, password_, db_;
+  uint16_t port_;
 
-  std::unique_ptr<boost::mysql::tcp_ssl_connection> conn_;
   std::unique_ptr<boost::asio::strand<boost::asio::io_context::executor_type>>
       strand_;
 
-  // 비동기 락 시스템을 위한 상태 변수
-  bool is_locked_ = false;
+  // 풀 상태 관리
+  std::vector<std::unique_ptr<boost::mysql::tcp_ssl_connection>>
+      available_conns_;
   std::queue<std::shared_ptr<boost::asio::steady_timer>> waiting_queue_;
+  const uint16_t max_pool_size_ = 10;
+  int16_t current_total_conns_ = 0;
 };