feat!: enable explicit setting of API base path and prefix

[morremeyer]

This renders the complicated and error-prone auto-detection obsolete.
It also enables the use of the swagger documentation to directly
interact with the API.

BREAKING CHANGE: URLs are no longer generated by autodetection.
You need to set the environment variable API_HOST_PROTOCOL and,
if the API is at a sub-path, API_BASE_PATH.

[codecov]

Codecov Report

Merging #246 (667a562) into main (2f34928) will decrease coverage by 0.61%.
The diff coverage is 77.08%.

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
- Coverage   97.18%   96.57%   -0.62%     
==========================================
  Files          15       16       +1     
  Lines        1316     1313       -3     
==========================================
- Hits         1279     1268      -11     
- Misses         26       34       +8     
  Partials       11       11              

Impacted Files	Coverage Δ
pkg/httputil/request.go	100.00% <ø> (+6.12%)	⬆️
internal/router/middleware.go	8.33% <8.33%> (ø)
internal/router/router.go	90.75% <100.00%> (+1.03%)	⬆️
pkg/controllers/account.go	100.00% <100.00%> (ø)
pkg/controllers/allocation.go	100.00% <100.00%> (ø)
pkg/controllers/budget.go	97.82% <100.00%> (ø)
pkg/controllers/category.go	97.93% <100.00%> (ø)
pkg/controllers/envelope.go	97.61% <100.00%> (ø)
pkg/controllers/transaction.go	100.00% <100.00%> (ø)

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