판매기능 추가

## 목적
주식 판매 기능
## 작업 상세 내용
자신이 가지고 있는 주식을 판매할 수 있다.
## Reference

The issue describes adding a stock selling feature. First, examine the existing codebase to understand the data structures for user holdings and the current buying functionality. Look for files related to transaction processing, user portfolios, and stock market logic. Determine how to validate a sale (e.g., sufficient holdings) and update the user's balance and portfolio. A test to verify the sale completes successfully would show the feature is done.

Written by the indexing model from the issue text.