Database connection trong Go best practices?

sql.Open() trả connection pool (không phải single connection).

• Set: db.SetMaxOpenConns(25), db.SetMaxIdleConns(5), db.SetConnMaxLifetime(5*time.Minute).
• KHÔNG defer db.Close() mỗi request vì sql.DB là long-lived pool — chỉ close khi app shutdown.
• Prepared statements cho queries lặp lại.
• Context timeout cho queries: db.QueryContext(ctx, query).