Add plan deletion, with a confirmation screen listing affected clubs

Due.plan is PROTECT -- a plan that has ever billed anyone can never
truly be removed, on purpose: amount/period_end/grace_until are
frozen on a Due precisely so a later change can't rewrite what was
actually charged, and losing the plan link off an old Due would do
exactly that to every historical invoice.

"Delete" therefore means one of two things, chosen automatically
(billing/services/plans.py):
- never billed anyone -> the row is removed outright.
- has billing history -> soft-deleted (Plan.deleted_at, is_active
  off): hidden from every picker/listing via the new opt-in
  Plan.objects.visible(), but the row survives so old invoices still
  show what they were billed under.

Either way, every club currently on the plan is unsubscribed outright
-- its Subscription row deleted, not just its plan field cleared.
"No plan" was already a fully-understood state everywhere else in the
app, so this reuses it instead of inventing a new one.

Also handles the easy-to-miss second group: a club on a DIFFERENT
plan, mid-trial, configured to convert to the plan being deleted
(Subscription.post_trial_plan). Left alone that would try to convert
onto a hidden/gone plan later; instead that club's trial is ended now
(both trial fields cleared, per the CheckConstraint requiring them
together) so it needs a new plan picked by hand.

The confirmation screen is a real page, not a modal like every other
billing action -- naming exactly which clubs are affected, in both
groups, and that list can be long.

controlpanel/views.py

@@ -7,6 +7,8 @@ from django.shortcuts import get_object_or_404, redirect
 from django.urls import reverse
 from django.utils import timezone
 from django.utils.formats import date_format
+from django.utils.translation import gettext_lazy as _
+from django.utils.translation import ngettext
 from django.views.generic import CreateView, DetailView, FormView, ListView, TemplateView, UpdateView, View
 from waffle import get_waffle_flag_model, get_waffle_switch_model
 
@@ -14,6 +16,7 @@ from billing.models import Due, Plan, PlanPrice
 from billing.services import BillingError
 from billing.services.dues import next_period_start, open_period, reactivate, record_payment, start_trial, subscribe, waive
 from billing.services.invoices import invoice_pdf, issue_invoice
+from billing.services.plans import delete_plan, plan_deletion_impact
 from club.models import Club, ClubRole
 from events.models import Location
 from features.models import Maintenance
@@ -401,7 +404,7 @@ class BillingView(PlatformStaffRequiredMixin, TemplateView):
         # Bound per-row so each "Edit" / "New price" modal can render its own form: the
         # template can't call PlanForm(instance=plan) itself, so the form rides along on
         # the object it belongs to.
-        plans = list(Plan.objects.prefetch_related("prices").annotate(club_count=Count("subscriptions")))
+        plans = list(Plan.objects.visible().prefetch_related("prices").annotate(club_count=Count("subscriptions")))
         for plan in plans:
             plan.edit_form = PlanForm(instance=plan)
             plan.price_form = PlanPriceForm()
@@ -448,6 +451,53 @@ class PlanUpdateView(PlatformStaffRequiredMixin, RedirectOnInvalidMixin, UpdateV
         return reverse("controlpanel:billing")
 
 
+class PlanDeleteView(PlatformStaffRequiredMixin, TemplateView):
+    """Confirm-then-delete for a plan. Unlike every other billing action, this is a real
+    page rather than a modal: the whole point is naming exactly which clubs lose their plan
+    (or lose their trial's scheduled landing plan), and that list can be long -- see
+    billing.services.plans for what "delete" actually does to a plan with billing history.
+    """
+
+    template_name = "controlpanel/plan_delete.html"
+
+    @property
+    def plan(self):
+        return get_object_or_404(Plan, pk=self.kwargs["pk"])
+
+    def get(self, request, *args, **kwargs):
+        plan = self.plan
+        if plan.is_deleted:
+            notify(request, f"i|{_('Already deleted')}|" + _("“%(plan)s” has already been deleted.") % {"plan": plan.name})
+            return redirect("controlpanel:billing")
+        return super().get(request, *args, **kwargs)
+
+    def get_context_data(self, **kwargs):
+        return super().get_context_data(nav="billing", plan=self.plan, impact=plan_deletion_impact(self.plan), **kwargs)
+
+    def post(self, request, *args, **kwargs):
+        plan = self.plan
+        plan_name = plan.name
+        impact = delete_plan(plan)
+
+        if impact.will_hard_delete:
+            body = _("“%(plan)s” has been deleted.") % {"plan": plan_name}
+        else:
+            body = _("“%(plan)s” has been deleted. It has billing history, so it's hidden rather than removed — past invoices still show it.") % {"plan": plan_name}
+        notify(request, f"w|{_('Plan deleted')}|{body}")
+
+        if impact.unsubscribed_clubs:
+            count = len(impact.unsubscribed_clubs)
+            club_body = ngettext("%(count)d club now has no plan.", "%(count)d clubs now have no plan.", count) % {"count": count}
+            notify(request, f"w|{_('Clubs affected')}|{club_body}")
+
+        if impact.broken_trial_clubs:
+            count = len(impact.broken_trial_clubs)
+            trial_body = ngettext("%(count)d club's trial lost its scheduled plan and needs a new one picked.", "%(count)d clubs' trials lost their scheduled plan and need a new one picked.", count) % {"count": count}
+            notify(request, f"w|{_('Trials affected')}|{trial_body}")
+
+        return redirect("controlpanel:billing")
+
+
 class PlanPriceCreateView(PlatformStaffRequiredMixin, RedirectOnInvalidMixin, CreateView):
     """A rate change is a new dated price, never an edit of the old one — periods already
     billed keep the amount they were billed at.