fix: redirect admitted users away from beta wait page

[Copilot]

Users granted beta access while sitting on /beta/wait/ would remain stuck there on reload — the view unconditionally rendered the waiting page regardless of current admission status.

Changes

• views.py — beta_wait: Call is_beta_admitted() before rendering; redirect to overview if the user is already admitted.

def beta_wait(request: HttpRequest) -> HttpResponse:
    if is_beta_admitted(request.user):
        return redirect(reverse("overview"))
    return render(request, "toxtempass/beta_wait.html")

• tests/test_admin_beta.py: Two new cases — pending user still sees the wait page; admitted user is redirected to overview.