chore: API improvements (#3637)

- Unique validations for Inbox members and Team member objects
- Move notification processing to Async

spec/controllers/api/v1/accounts/inbox_members_controller_spec.rb

@@ -81,7 +81,7 @@ RSpec.describe 'Inbox Member API', type: :request do
       end
 
       it 'add inbox members' do
-        params = { inbox_id: inbox.id, user_ids: [agent_to_add.id] }
+        params = { inbox_id: inbox.id, user_ids: [old_agent.id, agent_to_add.id] }
 
         post "/api/v1/accounts/#{account.id}/inbox_members",
              headers: administrator.create_new_auth_token,