Add platform billing: tiers, dues, payments and invoices

RosterChief charging the clubs, which is a different domain from `shop` (a club
charging its members). Nothing here is club-scoped: these rows reference a Club,
they are not owned by one, and no club user ever sees them.

- Tier + TierPrice. Prices are dated, not keyed by year: a rate change is one row
  with a future active_from, and price_on(day) answers "what was in force then".
  A tier with no price yet returns None, which callers must treat as "cannot
  bill" -- never as free.
- Due: one rolling-year period per club, with a 45-day grace tail. The tier and
  the amount are SNAPSHOTS taken when the period opens. Raise the price and last
  year's period must still say what was actually charged; reading it back through
  the tier would silently rewrite financial history.
- DuePayment: partial payments accumulate. amount_paid is re-summed from the
  payments on every change, never incremented -- an increment drifts the moment a
  payment is deleted, and the drift still looks like money.
- Invoice: PDF via WeasyPrint, rendered on demand from the frozen snapshot. Only
  the number is stored, in one platform-wide series (unlike the shop's per-club
  order numbers), and re-issuing returns the existing one rather than burning a
  number -- a gap in an invoice series is a question you don't want to answer.
  WeasyPrint is imported lazily: it binds to native pango/cairo, and the app, the
  tests and every other page must still run on a machine without them.
- archive_overdue_clubs reports by default and archives only with --commit. That
  asymmetry is deliberate: this switches off paying customers, so a bad clock or a
  cron misconfiguration should cost an email, not a morning of angry clubs. A club
  with auto_archive off is spared entirely.

Renewal continues from the last period end, not from the payment date: a club that
pays two months late has still used those two months.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

billing/management/commands/archive_overdue_clubs.py

@@ -0,0 +1,38 @@
+"""Archive clubs whose billing period has gone unpaid past its grace period.
+
+Reports by default and only acts with --commit. That asymmetry is the point: this command
+switches off paying customers, and a cron misconfiguration, a clock skew or a bad import
+should cost you a confusing email, not a morning of angry clubs.
+"""
+
+from django.core.management.base import BaseCommand
+from django.utils import timezone
+
+from billing.services.dues import archivable_clubs
+
+
+class Command(BaseCommand):
+    help = "Archive clubs that are unpaid past their grace period (dry run unless --commit)."
+
+    def add_arguments(self, parser):
+        parser.add_argument("--commit", action="store_true", help="Actually archive them. Without this the command only reports.")
+
+    def handle(self, *args, **options):
+        today = timezone.localdate()
+        overdue = list(archivable_clubs(today))
+
+        if not overdue:
+            self.stdout.write(self.style.SUCCESS("Nothing overdue past grace."))
+            return
+
+        for due in overdue:
+            days = (today - due.grace_until).days
+            self.stdout.write(f"{due.club} — {due.tier}, {due.balance} owed, grace ended {due.grace_until} ({days} day{'s'[: days != 1]} ago)")
+
+        if not options["commit"]:
+            self.stdout.write(self.style.WARNING(f"\nDry run: {len(overdue)} club(s) would be archived. Re-run with --commit to do it."))
+            return
+
+        for due in overdue:
+            due.club.archive()
+        self.stdout.write(self.style.SUCCESS(f"\nArchived {len(overdue)} club(s). Their data is kept; restoring re-opens billing."))