Rework platform billing: per-plan clocks, grace from period start

Implements BILLING.md. The architecture was sound -- snapshot-on-Due,
dated prices, asymmetric dry-run commands are all kept -- so this
fixes the three hardcoded assumptions rather than rewriting.

The real defect: grace ran from period_END, so an annual club used
the whole unpaid year plus 45 days (~410 days) before anything
switched it off. Grace now runs from the period START, and every
clock is per-plan.

- Tier -> Plan (+ TierPrice -> PlanPrice, and every FK). Migration
  0004 is hand-written: run non-interactively, makemigrations emits
  DeleteModel+CreateModel and drops every price, subscription and
  due. Its two RemoveConstraints must come first, or SQLite's
  table-rebuild tries to render a constraint over a just-renamed
  column. Verified by round-tripping real rows through it.
- Plan gains duration_months / renewal_lead_days / grace_days /
  is_trial, with CheckConstraints and a matching clean() so the form
  reports an impossible plan instead of 500ing on IntegrityError.
- Existing dues keep their stored grace_until. Re-deriving it would
  put the date in the past for every open annual period and archive
  the entire paying customer base on the next --commit run.
- Trials take their length from the trial plan's own duration_months;
  start_trial() loses its trial_months argument.
- New BillingNotice service drives a club-facing warning: every level
  on the dashboard, and on every management page once urgent.
- send_billing_reminders emails club admins, once per escalation
  level so a daily cron is not a daily email. SMTP settings are
  env-driven and provider-agnostic; the backend defaults to console.
- Paying does not auto-restore an archived club -- the control panel
  surfaces a Reactivate prompt instead, since a club can also be
  archived by hand.

controlpanel/urls.py

@@ -25,9 +25,9 @@ urlpatterns = [
     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/plans/new/", views.PlanCreateView.as_view(), name="plan_create"),
+    path("billing/plans/<uuid:pk>/edit/", views.PlanUpdateView.as_view(), name="plan_update"),
+    path("billing/plans/<uuid:pk>/prices/new/", views.PlanPriceCreateView.as_view(), name="plan_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"),