feat: add POST method for /v3/transactions

[morremeyer]

With this, API v3 supports transaction creation.

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (0878cda) 81.52% compared to head (2574d94) 81.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #838      +/-   ##
==========================================
+ Coverage   81.52%   81.59%   +0.06%     
==========================================
  Files          42       42              
  Lines        4282     4336      +54     
==========================================
+ Hits         3491     3538      +47     
- Misses        653      659       +6     
- Partials      138      139       +1     

Files	Coverage Δ
pkg/controllers/account_v1.go	80.00% <100.00%> (ø)
pkg/controllers/allocation.go	88.67% <100.00%> (ø)
pkg/controllers/category.go	88.88% <100.00%> (ø)
pkg/controllers/envelope.go	88.35% <100.00%> (ø)
pkg/controllers/month.go	86.84% <100.00%> (ø)
pkg/controllers/month_config.go	91.12% <100.00%> (ø)
pkg/controllers/transaction_v1.go	91.38% <100.00%> (ø)
pkg/controllers/transaction_v2.go	88.46% <100.00%> (ø)
pkg/httperrors/errors.go	90.22% <ø> (ø)
pkg/httputil/request.go	75.00% <100.00%> (+10.29%)	⬆️
... and 5 more

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.