Remove UUID requirement from push notification device registration

- Remove UUID column from devices table entirely
- Update unique index from (owner, uuid) to (owner, token)
- Simplify create action to just create device records
- Add token-based unregister route for API clients
- Consolidate error handling with rescue_from
- Update fixtures to remove uuid references

Devices are now identified by (owner, token) instead of UUID.
This simplifies the client-side registration flow.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

saas/lib/fizzy/saas/engine.rb

@@ -45,7 +45,9 @@ module Fizzy
           end
 
           namespace :users do
-            resources :devices, only: [ :index, :create, :destroy ]
+            resources :devices, only: [ :index, :create, :destroy ] do
+              delete :destroy, on: :collection, as: :unregister
+            end
           end
         end
       end