fixup! Update `published` when publishing a post

This is a follow-up to 094f35da, which unfortunately didn't work as is, because decorators aren't triggered.

Instead, we need to actually replace the published property with a newly generated published value.

This requires hooking in the Clock, which we should expose as a bean, instead of it being defaulted, as it allows us to centrally manage time in other places, too, as well as having the benefit of Dependency Injection for tests.