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/send_billing_reminders.py

@@ -0,0 +1,64 @@
+"""Email club admins about platform fees they owe.
+
+Reports by default and only sends with --commit, the same posture as archive_overdue_clubs
+and for a related reason: this one mails paying customers, and a bad clock, a bad import or a
+rehearsal against production data should cost you a confusing dry-run listing rather than a
+mailshot you cannot recall.
+
+Idempotent across runs by design -- one reminder per due per escalation level, tracked on
+Due.last_reminder_level -- so a daily cron does not produce a daily email.
+"""
+
+from django.core.management.base import CommandError
+
+from billing.services.reminders import reminders_to_send, send_reminder
+from club.models import Club
+from features.commands import MaintenanceAwareCommand
+
+
+class Command(MaintenanceAwareCommand):
+    help = "Email club admins about outstanding platform fees (dry run unless --commit)."
+
+    def add_arguments(self, parser):
+        parser.add_argument("--commit", action="store_true", help="Actually send. Without this the command only reports.")
+        parser.add_argument("--force", action="store_true", help="Re-send even where a reminder already went out at this level.")
+
+    def handle(self, *args, **options):
+        clubs = Club.objects.active().select_related("subscription", "subscription__plan").order_by("name")
+        results = reminders_to_send(clubs, force=options["force"])
+
+        sendable = [result for result in results if result.sent]
+        skipped = [result for result in results if not result.sent]
+
+        if not results:
+            self.stdout.write(self.style.SUCCESS("Nothing owing. No reminders to send."))
+            return
+
+        for result in skipped:
+            style = self.style.WARNING if result.recipients else self.style.ERROR
+            self.stdout.write(style(f"{result.club} — skipped: {result.skipped_reason}"))
+
+        for result in sendable:
+            self.stdout.write(f"{result.notice.level:>7} · {result.club} — €{result.notice.amount_outstanding} owed, {result.notice.days_until_archive}d to archive → {', '.join(result.recipients)}")
+
+        if not options["commit"]:
+            self.stdout.write(self.style.WARNING(f"\nDry run: {len(sendable)} reminder(s) would be sent. Re-run with --commit to send them."))
+            return
+
+        failures = []
+        for result in sendable:
+            try:
+                send_reminder(result.club, result.notice, recipients=result.recipients)
+            except OSError as error:
+                # One bad address or a momentary SMTP failure must not stop the rest of the
+                # run: the clubs further down the list are the ones closest to being archived.
+                failures.append(f"{result.club}: {error}")
+                self.stdout.write(self.style.ERROR(f"{result.club} — {error}"))
+
+        sent = len(sendable) - len(failures)
+        self.stdout.write(self.style.SUCCESS(f"\nSent {sent} reminder(s)."))
+
+        if failures:
+            # Non-zero so cron mails you: a club that could not be warned is a club that gets
+            # archived without notice.
+            raise CommandError(f"{len(failures)} reminder(s) could not be sent:\n  " + "\n  ".join(failures))