Manage billing from the control panel

A Billing tab (tiers, their dated prices, and everything we are owed), a billing
panel on each club (plan, periods, payment history, invoice), and the dues on the
dashboard and the club tables.

Every state change goes through the billing service, and a BillingError surfaces
as a message rather than a 500 -- so "that period is waived", "no price in force",
"already billed for that period" and a missing PDF library all explain themselves
instead of crashing.

The dashboard now separates the two pots of money that were previously one word.
"Revenue per month" was CLUB SHOP revenue -- members paying their clubs, which is
never ours -- sitting on our dashboard under a label that implied it was income.
It is now "Platform dues per month" (what clubs paid us) with the club-shop series
renamed club_revenue, and the club tables carry a Plan column and what each club
owes us, annotated in the same single query.

Rate changes are add-only in the UI as well as the model: the price form creates a
dated row and never edits the last one, and a test asserts that raising the rate
leaves an already-open period at the amount it was billed at.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

controlpanel/urls.py

@@ -21,6 +21,16 @@ urlpatterns = [
     path("features/flags/new/", views.FlagCreateView.as_view(), name="flag_create"),
     path("features/flags/<int:pk>/edit/", views.FlagUpdateView.as_view(), name="flag_update"),
     path("features/switches/<int:pk>/toggle/", views.SwitchToggleView.as_view(), name="switch_toggle"),
+    # Billing (platform charging the clubs)
+    path("billing/", views.BillingView.as_view(), name="billing"),
+    path("billing/tiers/new/", views.TierCreateView.as_view(), name="tier_create"),
+    path("billing/tiers/<uuid:pk>/edit/", views.TierUpdateView.as_view(), name="tier_update"),
+    path("billing/tiers/<uuid:pk>/prices/new/", views.TierPriceCreateView.as_view(), name="tier_price_create"),
+    path("billing/dues/<uuid:pk>/pay/", views.RecordPaymentView.as_view(), name="due_pay"),
+    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>/period/new/", views.OpenPeriodView.as_view(), name="club_open_period"),
     # Platform admins (superusers only)
     path("admins/", views.PlatformAdminListView.as_view(), name="admins"),
     path("admins/add/", views.PlatformAdminAddView.as_view(), name="admin_add"),