feat(core): add context-based graceful shutdown on interrupt signals

[JosephKav]

Introduces context handling to ensure DB, web server and saving can shutdown safely on user interrupts, or container stop signals.

[codecov]

Codecov Report

❌ Patch coverage is 90.21526% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.47%. Comparing base (6ea0849) to head (bfea880).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/argus/main.go	69.24%	9 Missing and 7 partials ⚠️
config/save.go	86.12%	12 Missing and 3 partials ⚠️
db/init.go	87.31%	6 Missing and 2 partials ⚠️
config/settings.go	80.56%	0 Missing and 7 partials ⚠️
util/verify.go	80.00%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #766      +/-   ##
==========================================
- Coverage   98.83%   98.47%   -0.36%     
==========================================
  Files         132      135       +3     
  Lines        8906     9093     +187     
==========================================
+ Hits         8801     8953     +152     
  Misses         80       80              
- Partials       25       60      +35     

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