Add trial subscriptions with automatic switch to a pre-selected plan

A club with no subscription yet can be started on a short trial (e.g.
2 months) from the control panel, on a tier picked up front for what
it switches to once the trial ends -- no manual follow-up needed. The
trial is a real billed period on a dedicated trial tier, reusing the
existing invoice/grace/archive machinery unchanged; the switch happens
in open_period() itself so it fires whether reached via the scheduled
renewal command or a platform admin's manual "Open period" click.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R1gj3J1QPfP38XWpnpbFpy

controlpanel/urls.py

@@ -32,6 +32,7 @@ urlpatterns = [
     path("billing/dues/<uuid:pk>/waive/", views.WaiveDueView.as_view(), name="due_waive"),
     path("billing/dues/<uuid:pk>/invoice.pdf", views.InvoicePdfView.as_view(), name="due_invoice"),
     path("clubs/<uuid:pk>/subscription/", views.SubscribeClubView.as_view(), name="club_subscribe"),
+    path("clubs/<uuid:pk>/trial/start/", views.ClubStartTrialView.as_view(), name="club_trial_start"),
     path("clubs/<uuid:pk>/period/new/", views.OpenPeriodView.as_view(), name="club_open_period"),
     # Platform admins (superusers only)
     path("admins/", views.PlatformAdminListView.as_view(), name="admins"),