Small native API changes: include identity ID, return empty signup completion body

Include identity.id in the my/identity.json response and return an
empty body from the CREATE signup/completions.json endpoint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/controllers/signups/completions_controller.rb

@@ -29,7 +29,7 @@ class Signups::CompletionsController < ApplicationController
           redirect_to landing_url(script_name: @signup.account.slug)
         end
 
-        format.json { render json: { account_id: @signup.account.id }, status: :created }
+        format.json { head :created }
       end
     end