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>

controlpanel/templates/controlpanel/club_detail.html

@@ -234,6 +234,11 @@
             {% else %}
                 <p class="text-sm opacity-70">
                     On <strong>{{ subscription.tier.name }}</strong>.
+                    {% if subscription.auto_renew %}
+                        Renews automatically 30 days before the period ends.
+                    {% else %}
+                        <span class="badge badge-warning badge-sm">Auto-renew off</span> — you must open each period by hand, or this club uses the platform for free.
+                    {% endif %}
                     {% if subscription.auto_archive %}
                         Archived automatically when a period goes unpaid past its grace period.
                     {% else %}