fix: disallow negative ttls

[pjenvey]

Closes #1276

[codecov-io]

Codecov Report

Merging #1283 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1283   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          59      59           
  Lines        9531    9604   +73     
======================================
+ Hits         9531    9604   +73

Impacted Files	Coverage Δ
autopush/web/webpush.py	100% <100%> (ø)	⬆️
autopush/tests/test_integration.py	100% <100%> (ø)	⬆️
autopush/tests/test_diagnostic_cli.py	100% <0%> (ø)	⬆️
autopush/diagnostic_cli.py	100% <0%> (ø)	⬆️

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