chore: Use installation config URL for captain app URL (#9874)

Use `InstallationConfig.where(name: 'CAPTAIN_APP_URL')` instead of
`ENV.fetch('CAPTAIN_APP_URL', '')`

app/controllers/api/v1/accounts/integrations/captain_controller.rb

@@ -8,7 +8,9 @@ class Api::V1::Accounts::Integrations::CaptainController < Api::V1::Accounts::Ba
       "&email=#{URI.encode_www_form_component(@hook['settings']['account_email'])}" \
       "&account_id=#{URI.encode_www_form_component(@hook['settings']['account_id'])}"
 
-    sso_url = "#{ENV.fetch('CAPTAIN_APP_URL', '')}/sso?#{params_string}"
+    installation_config = InstallationConfig.find_by(name: 'CAPTAIN_APP_URL')
+
+    sso_url = "#{installation_config.value}/sso?#{params_string}"
     render json: { sso_url: sso_url }, status: :ok
   end