Restrict backend to API-only mode

[lfilmeyer]

See https://cfb-public.slack.com/archives/C03TE14E3RP/p1722385703058509

The backend app is intended to be API-only and just serve JSON, not HTML. For instance, when a 500 error occurs, we currently serve an HTML error response.

[mzagaja]

Mike tried:

-class ApplicationController < ActionController::Base
+class ApplicationController < ActionController::API
   # before_action :authenticate_user!
   include Pundit::Authorization
   after_action :verify_authorized, except: :index
diff --git a/backend/config/application.rb b/backend/config/application.rb
index cc61c07..06461ea 100644
--- a/backend/config/application.rb
+++ b/backend/config/application.rb
@@ -39,6 +39,9 @@ module UrbanLeagueHeatPumpAccelerator
     # Configure Active Job to use sucker_punch backend
     config.active_job.queue_adapter = :sucker_punch
 
+    config.api_only = true
+    config.debug_exception_response_format = :api
+
     config.autoload_paths += %W[#{config.root}/lib]

and it caused some tests to break.