refactor: add thresholds to variable model

[eatyourgreens]

Add variable.lower_threshold and variable.upper_threshold to the variable model, so that thresholds are saved in the app database.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.52%. Comparing base (c227f57) to head (872ea54).
Report is 24 commits behind head on ui-enhancements.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           ui-enhancements     #584      +/-   ##
===================================================
+ Coverage            76.17%   76.52%   +0.34%     
===================================================
  Files                  108      110       +2     
  Lines                 5600     5645      +45     
===================================================
+ Hits                  4266     4320      +54     
+ Misses                1334     1325       -9     

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[sonarqubecloud]

Quality Gate passed for 'pkpdapp'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
90.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate failed for 'pkpdapp-team_pkpdapp_frontend'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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