perf: offload BeautifulSoup parsing to a thread via asyncio.to_thread

[vdusek]

Summary

• BeautifulSoup() constructor performs synchronous HTML parsing that blocks the event loop, degrading concurrency.
• Wrapped parse() and parse_text() in asyncio.to_thread() to offload parsing to a worker thread, consistent with how ParselParser.parse() already works.

Test plan

• All 78 existing BeautifulSoup crawler tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.23%. Comparing base (02a18ea) to head (87a846b).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1817      +/-   ##
==========================================
- Coverage   92.23%   92.23%   -0.01%     
==========================================
  Files         157      157              
  Lines       10863    10884      +21     
==========================================
+ Hits        10020    10039      +19     
- Misses        843      845       +2     

Flag	Coverage Δ
unit	92.23% <100.00%> (-0.01%)	⬇️

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[Mantisus]

LGTM