API spec

[hillalex]

Closes #16 by adding a full openapi spec.

Also some opportunistic refactoring to make code better organised.

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.37%. Comparing base (36abddb) to head (7b49e63).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
R/router.R	88.88%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   94.25%   94.37%   +0.11%     
==========================================
  Files           5        6       +1     
  Lines         383      391       +8     
==========================================
+ Hits          361      369       +8     
  Misses         22       22              

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