FI-3487: Fix response content types

[Jammjammjamm]

Summary

This branch fixes some JSON API responses which didn't have a JSON content-type. It also fixes the content-type in a few other routes.

Testing Guidance

Open the network tab in your browser inspector. On main, you will see that the responses to some api requests have a content-type of plain or octet-stream. On this branch, the responses should all have correct content types.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.71%. Comparing base (3074d02) to head (da72be8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/inferno/apps/web/router.rb	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #616   +/-   ##
=======================================
  Coverage   84.70%   84.71%           
=======================================
  Files         283      283           
  Lines       12398    12401    +3     
  Branches     1503     1503           
=======================================
+ Hits        10502    10505    +3     
  Misses       1888     1888           
  Partials        8        8           

Flag	Coverage Δ
backend	92.33% <88.88%> (+<0.01%)	⬆️
frontend	79.20% <ø> (ø)

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