Document API with OpenAPI spec (API 1.1)

[krestenlaust]

In order to make the use of the API more welcoming, the API should be formally described as an OpenAPI specification.

Endpoints:

• api/member
• api/member/balance
• api/member/get_id
• api/member/active
• api/member/sales
• api/member/payment/qr
• api/products/named_products
• api/products/active_products
• api/products/category_mappings
• api/sale

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.83%. Comparing base (294159e) to head (507688b).
Report is 9 commits behind head on next.

❗ Current head 507688b differs from pull request most recent head 51ba904

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Additional details and impacted files

@@             Coverage Diff             @@
##             next     #420       +/-   ##
===========================================
+ Coverage   53.49%   80.83%   +27.33%     
===========================================
  Files          33       33               
  Lines        4621     3047     -1574     
  Branches      274      255       -19     
===========================================
- Hits         2472     2463        -9     
+ Misses       2097      545     -1552     
+ Partials       52       39       -13     

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[AshesOfEther]

Looks mostly good. I suggest using the sample instance of Swagger UI to check that it's right. Perhaps we could publish that on a docs site of our own? That's out of scope for this PR though.