fix: correct SNR_threshold type annotation and xarray scalar extraction

[beatfactor]

• clean/api.py: Change SNR_threshold parameter type hint from float to str to match its default value '3.0dB'
• calibrate/cal_params.py: Use .values.item() instead of .data.tolist() for xarray scalar extraction (forward-compatible API)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.56%. Comparing base (a7c2a62) to head (88c6c53).
⚠️ Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1622      +/-   ##
==========================================
+ Coverage   84.90%   85.56%   +0.65%     
==========================================
  Files          79       79              
  Lines        6998     6998              
==========================================
+ Hits         5942     5988      +46     
+ Misses       1056     1010      -46     

Flag	Coverage Δ
unittests	85.56% <100.00%> (+0.65%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.