Auto-renew subscriptions before they lapse

Answers "does a plan renew itself?": until now, no — and that was a silent revenue
leak, not merely a missing convenience. A club whose period ended with its last due
PAID owes nothing, so dues_overdue() is empty, so archive_overdue_clubs never fires.
The club kept using the platform for free and no dashboard number went red, because
nothing was ever billed. The safety net only caught clubs you remembered to invoice.

`renew_subscriptions` (cron) issues the next period 30 days before the current one
ends, so the invoice lands before the period lapses and grace only matters for
genuine non-payers. It is idempotent by construction: a just-renewed club has a
latest period a year out, past the horizon, so a second run is a no-op.

It ACTS by default and previews with --dry-run — the opposite asymmetry to
archiving, and deliberately so. Archiving switches off a customer, so not-acting is
safe there; here, not-acting is the expensive failure, because an unbilled club is
also an unchased one. An unpriced tier fails that one club loudly (non-zero exit, so
cron mails you) without stopping the rest.

Opt-out per club via Subscription.auto_renew, mirroring auto_archive: off means you
invoice that club by hand. The dashboard gains a "renewals pending" count that
should sit at ~0 — a number here means cron has died and a club is about to go free,
which no other metric would reveal.

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

billing/models.py

@@ -22,6 +22,11 @@ ZERO = Decimal("0.00")
 #: A club stays live for six weeks past the end of an unpaid period before it is archived.
 GRACE_DAYS = 45
 
+#: The next period is issued this long before the current one ends, so the invoice reaches the
+#: club — and can be paid — before the old period lapses. Grace then only matters for genuine
+#: non-payers, rather than for everyone who takes a fortnight to pay a bank transfer.
+RENEWAL_LEAD_DAYS = 30
+
 
 def add_one_year(day: date) -> date:
     """The day one year on. 29 February has no counterpart in a common year, so it falls
@@ -93,6 +98,7 @@ class Subscription(UUIDModel):
 
     club = models.OneToOneField("club.Club", on_delete=models.CASCADE, related_name="subscription", verbose_name=_("club"))
     tier = models.ForeignKey(Tier, on_delete=models.PROTECT, related_name="subscriptions", verbose_name=_("tier"))
+    auto_renew = models.BooleanField(_("auto renew"), default=True, help_text=_("Issue the next period automatically before this one ends. Off means you invoice this club by hand."))
     auto_archive = models.BooleanField(_("auto archive"), default=True, help_text=_("Archive this club when a period goes unpaid past its grace period."))
     notes = models.TextField(_("notes"), blank=True)