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.

billing/management/commands/renew_subscriptions.py

@@ -9,7 +9,6 @@ fires either. A missed renewal is silent, and silence is the expensive failure.
 
 from django.core.management.base import CommandError
 
-from billing.models import RENEWAL_LEAD_DAYS
 from billing.services import BillingError
 from billing.services.dues import renew, subscriptions_due_for_renewal
 from features.commands import MaintenanceAwareCommand
@@ -20,7 +19,7 @@ class Command(MaintenanceAwareCommand):
 
     def add_arguments(self, parser):
         parser.add_argument("--dry-run", action="store_true", help="Report what would be issued, and issue nothing.")
-        parser.add_argument("--lead-days", type=int, default=RENEWAL_LEAD_DAYS, help=f"Issue this many days before the period ends (default {RENEWAL_LEAD_DAYS}).")
+        parser.add_argument("--lead-days", type=int, default=None, help="Override every plan's own renewal lead. Left off, each plan uses its own.")
 
     def handle(self, *args, **options):
         due_for_renewal = subscriptions_due_for_renewal(lead_days=options["lead_days"])
@@ -34,13 +33,13 @@ class Command(MaintenanceAwareCommand):
             club = subscription.club
 
             if options["dry_run"]:
-                self.stdout.write(f"would renew {club} — {subscription.tier}, current period ends {subscription.latest_period_end or 'never opened'}")
+                self.stdout.write(f"would renew {club} — {subscription.plan}, current period ends {subscription.latest_period_end or 'never opened'}")
                 continue
 
             try:
                 due = renew(subscription)
             except BillingError as error:
-                # One unpriced tier must not stop every other club from being billed.
+                # One unpriced plan must not stop every other club from being billed.
                 failures.append(f"{club}: {error}")
                 self.stdout.write(self.style.ERROR(f"{club} — {error}"))
                 continue