Golang Unit Test Coverage for usage of github.com/golang-jwt/jwt/v4 in backend/middleware/*

[pwdel]

• golang-jwt/jwt/v4 is used to issue tokens for sign-in.
• Test coverage for what this package issues would be helpful in case of change.
• Alternatively, if we choose instead to use cookies rather than tokens (which is recommended) then we can convert this ticket to create login with cookies and associated test.

https://github.com/orgs/openpredictionmarkets/projects/4/views/1?pane=issue&itemId=60458786