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.

management/views.py

@@ -9,7 +9,7 @@ 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 billing.models import RENEWAL_LEAD_DAYS, Due
+from billing.models import Due
 from club.mixins import (
     ClubAdminRequiredMixin,
     ClubStaffRequiredMixin,
@@ -86,10 +86,13 @@ class HomeView(ClubStaffRequiredMixin, TemplateView):
         club, user = self.request.club, self.request.user
         subscription = getattr(club, "subscription", None)
 
+        # "Your period ends soon" is a different question from "you owe us money", and only
+        # worth raising when nothing is owed -- an unpaid club gets the billing notice from
+        # the context processor instead, which is louder and more urgent.
         billing_ends_at = None
-        if subscription is not None:
+        if subscription is not None and not club.dues.filter(status__in=Due.OWING).exists():
             latest_due = club.dues.exclude(status=Due.Status.CANCELLED).order_by("-period_end").first()
-            if latest_due is not None and 0 <= (latest_due.period_end - timezone.localdate()).days <= RENEWAL_LEAD_DAYS:
+            if latest_due is not None and 0 <= (latest_due.period_end - timezone.localdate()).days <= subscription.plan.renewal_lead_days:
                 billing_ends_at = latest_due.period_end
 
         upcoming_events = scoped_to_managed_teams(Event.objects.filter(club=club, start__gte=timezone.now()), user, club).order_by("start").prefetch_related("teams")[:5]