refactor: use run_in_threadpool for asset service calls in watchlist router

[vcfgv]

📝 Pull Request Template

1. Related Issue

Closes #219

Type of Change (select one)

Performance

3. Description

Wrapped blocking service calls in the watchlist router with Starlette’s run_in_threadpool to avoid event-loop blocking caused by synchronous underlying libraries (e.g., yfinance).
Added the import and updated these async routes:

• GET /asset/search
• GET /asset/{ticker}
• GET /asset/{ticker}/price
• GET /{watchlist_name}
• GET /asset/{ticker}/price/historical.

4. Testing

• I have tested this locally.
• I have updated or added relevant tests.

5. Checklist

• I have read the Code of Conduct
• I have followed the Contributing Guidelines
• My changes follow the project's coding style