fix(risk): lower low and very_low FWI confidence thresholds

[MateoLostanlen]

• The current thresholds filter out too many alerts on low risk days, which is a problem in winter.
• Temporarily set low to 0.30 and very_low to 0.35 until the Ocorus study gives final values.
• Update the risk filter tests to match the new thresholds.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.87%. Comparing base (c203470) to head (1695bbb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #697      +/-   ##
==========================================
+ Coverage   91.30%   93.87%   +2.56%     
==========================================
  Files           3       59      +56     
  Lines         138     3214    +3076     
==========================================
+ Hits          126     3017    +2891     
- Misses         12      197     +185     

Flag	Coverage Δ
backend	93.98% <ø> (?)
client	91.30% <ø> (ø)

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