Async HTTP — replace ThreadPoolExecutor with asyncio + httpx

[dfdezdom]

Description

Replace the current ThreadPoolExecutor approach with asyncio + httpx for non-blocking concurrent fetching.

Acceptance criteria

• recommend() uses asyncio internally or exposes an async API alongside sync
• yfinance calls wrapped in async (via httpx or aiohttp)
• Performance at least as good as current ThreadPoolExecutor
• Backwards-compatible sync API maintained

Notes

Medium-to-high effort. yfinance is synchronous internally, so we would need to either use asyncio.to_thread() or switch to direct API calls via httpx. Worth evaluating if yfinance rate limits are a bottleneck.