Design opt-in template updates for presentations

Authors publish versions of presentation templates. A user creates a presentation from one version and customizes its slides and some styles. When the author publishes a newer version, every affected user is told, can preview what would change, and accepts or dismisses the update. Accepting must keep the user's compatible customizations and report conflicts rather than dropping them, and must be safe to retry. Design the data model (inherited values versus user overrides), the services and APIs, the event flow that finds affected presentations, and recovery when notifications are delayed.