Add maintenance mode: lock the platform down from the control panel

Closes every club subdomain with a 503 in that club's own colours, stands the
scheduled jobs down, and keeps open exactly what is needed to end it again.

The exemptions ARE the feature:

- /accounts/ stays open on the base domain. Close it too and you cannot sign in to
  turn maintenance off -- a lock-down with no key, fixable only from a shell.
- /healthz answers on every host. Close it and the load balancer decides the node
  is dead, stops routing to it, and takes the control panel down with everything
  else.
- migrate and collectstatic are NOT blocked. Maintenance is usually declared in
  order to run them; a blanket guard on BaseCommand would mean turning the mode off
  to do the work you turned it on for. Only the domain jobs (archive_overdue_clubs,
  extend_event_series, import_members_csv) refuse, and they exit non-zero so cron
  mails you -- a scheduled job that silently skips itself is how a month of billing
  goes missing.

The state is cached with a 10-second TTL, not for ever. Write-through makes the
flip instant for the shared Redis of a real deployment, and the TTL is the belt to
that braces: on a per-process cache -- a dev box with no Redis, or a misconfigured
deploy -- a lock-down that reached only one gunicorn worker would be worse than
useless. Live-verified: a club subdomain, its login page and the base domain all
503 while the control panel and the sign-in screens stay up.

Also adds the two deployment pieces asked for: compose.behind-proxy.yaml for a
dev/test box that already runs Caddy on :80 (app on the loopback, host Caddy proxies
to it -- and the host's Caddy still needs the DNS plugin, because the wildcard is
still a wildcard), and deploy/backup.sh + restore-check.sh with a cron schedule. The
backup writes to a .part file and only lands it once gzip -t says it is readable: a
truncated dump that looks like a backup is the failure you find on the day you need
it. The weekly restore rehearsal is the only line in that cron that proves the rest
work.

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

controlpanel/views.py

@@ -13,8 +13,9 @@ from billing.services import BillingError
 from billing.services.dues import next_period_start, open_period, reactivate, record_payment, subscribe, waive
 from billing.services.invoices import invoice_pdf, issue_invoice
 from club.models import Club, ClubRole
+from features.models import Maintenance
 
-from .forms import ClubAdminForm, ClubForm, DuePaymentForm, FlagForm, OpenPeriodForm, PlatformAdminForm, SubscriptionForm, TierForm, TierPriceForm
+from .forms import ClubAdminForm, ClubForm, DuePaymentForm, FlagForm, MaintenanceForm, OpenPeriodForm, PlatformAdminForm, SubscriptionForm, TierForm, TierPriceForm
 from .mixins import PlatformStaffRequiredMixin, PlatformSuperuserRequiredMixin
 from .services.admins import grant_club_admin, revoke_club_admin
 from .services.platform_admins import (
@@ -197,10 +198,28 @@ class FeatureListView(PlatformStaffRequiredMixin, TemplateView):
             nav="features",
             flags=Flag.objects.prefetch_related("clubs").order_by("name"),
             switches=Switch.objects.order_by("name"),
+            maintenance=Maintenance.current(),
+            maintenance_form=MaintenanceForm(),
             **kwargs,
         )
 
 
+class MaintenanceView(PlatformStaffRequiredMixin, View):
+    """Close the platform, or open it again."""
+
+    def post(self, request):
+        if Maintenance.is_on():
+            Maintenance.stop()
+            messages.success(request, "Maintenance ended. The clubs are back.")
+        else:
+            form = MaintenanceForm(request.POST)
+            message = form.cleaned_data["message"] if form.is_valid() else ""
+            Maintenance.start(message=message, user=request.user)
+            messages.warning(request, "Platform closed. Every club subdomain now serves a maintenance page, and the scheduled jobs stand down.")
+
+        return redirect("controlpanel:features")
+
+
 class FlagCreateView(PlatformStaffRequiredMixin, CreateView):
     model = Flag
     form_class = FlagForm