feat(notifications): support delayed sending

[piksel]

Fixes #1135

[codecov]

Codecov Report

Merging #1142 (dd663c1) into main (b4a225c) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1142      +/-   ##
==========================================
+ Coverage   61.81%   61.94%   +0.12%     
==========================================
  Files          23       23              
  Lines        1477     1482       +5     
==========================================
+ Hits          913      918       +5     
  Misses        478      478              
  Partials       86       86              

Impacted Files	Coverage Δ
pkg/notifications/email.go	90.90% <100.00%> (+0.21%)	⬆️
pkg/notifications/notifier.go	72.00% <100.00%> (+1.78%)	⬆️
pkg/notifications/shoutrrr.go	73.41% <100.00%> (+0.34%)	⬆️

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