fix(pushover): allow negative priority values

[piksel]

Fixes #204

[codecov]

Codecov Report

Merging #205 (0a9ac01) into main (2951098) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   76.87%   76.95%   +0.07%     
==========================================
  Files          85       85              
  Lines        2703     2703              
==========================================
+ Hits         2078     2080       +2     
+ Misses        442      441       -1     
+ Partials      183      182       -1     

Impacted Files	Coverage Δ
pkg/services/pushover/pushover.go	74.19% <100.00%> (+6.45%)	⬆️

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