### Changes

This PR:
- pushes PromEx to the bottom of supervision stack to avoid Endpoint
instrumentation failure
- ensures the site cache is ready by exposing it through the health
check endpoint
- fixes event timestamps being calculated at compile time, with
regression unit and integration tests

### Tests
- [x] Automated tests have been added
- [ ] This PR does not require tests

### Changelog
- [ ] Entry has been added to changelog
- [x] This PR does not make a user-facing change

### Documentation
- [ ] [Docs](https://github.com/plausible/docs) have been updated
- [x] This change does not need a documentation update

### Dark mode
- [ ] The UI has been tested both in dark and light mode
- [x] This PR does not change the UI