Try to calculate HR in zones based on moving time, not elapsed #40

[stancld]

Closes #40.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.

Project coverage is 62.82%. Comparing base (dc5d34b) to head (2e47cd4).

Files with missing lines	Patch %	Lines
backend/api/hr_processing.py	95.65%	1 Missing ⚠️
backend/api/models.py	0.00%	1 Missing ⚠️
backend/api/worker.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   62.63%   62.82%   +0.19%     
==========================================
  Files          16       16              
  Lines        1365     1372       +7     
==========================================
+ Hits          855      862       +7     
  Misses        510      510              

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