Correction to the QA root cause: the submit Button has carried
`disabled={createMutation.isPending}` since 3424bd22, and it does disable
synchronously after the first click (validateForm's setErrors forces a
re-render that re-reads the mutation snapshot), so an ordinary double-click
could not by itself produce two POSTs.
What was actually missing is a re-entrancy guard in handleSubmit, so any
submission that never touches the button -- implicit form submission, a
programmatic requestSubmit, or two submit events dispatched in one task,
which is the likely shape of the QA repro -- still fired two mutate() calls
racing the same computed slug, one of which 500'd on events_slug_unique.
Add isSubmittingRef (matching the isMountedRef idiom already in this file),
cleared in onSettled. Test proves 2 submit events -> 1 POST.
Not done: turning the backend's raw 500 on events_slug_unique into a
graceful "event already exists" 409. That is an adminEvents.js change and a
separate concern from the client-side race.
Refs testplan REPORT.md #6 (Part 7.03).
Paul Nothaft
c19e944b99