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/context_processors.py

@@ -9,6 +9,7 @@ action rather than the whole section (``NewsAuthorRequiredMixin``/``can_add_news
 
 from waffle import flag_is_active
 
+from billing.services.notices import club_billing_notice
 from club.services.access import can_add_news, has_management_access, is_club_admin, is_coach_manager
 
 #: Every management URL name, mapped to the nav item it should light up --
@@ -123,6 +124,23 @@ def is_admin(request):
     return {"is_club_admin": is_club_admin(request.user, club)}
 
 
+def billing_notice(request):
+    """What this club owes the platform, for the club's own admins.
+
+    A context processor rather than view context because the notice has to be able to follow
+    an admin onto every management page once it turns urgent -- billing/base.html renders it
+    at error level only, and the home page renders it at every level.
+
+    Admins only: platform billing is none of an ordinary member's business, and the query is
+    skipped entirely for everyone else rather than fetched and hidden in the template.
+    """
+    club = getattr(request, "club", None)
+    if club is None or not request.user.is_authenticated or not is_club_admin(request.user, club):
+        return {"billing_notice": None}
+
+    return {"billing_notice": club_billing_notice(club)}
+
+
 def management_position(request):
     """Whether the signed-in user holds a management position (or is ADMIN) --
     gates the nav's Locations/Opponents links, which ``ManagementPositionRequiredMixin``