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/tests.py

@@ -20,6 +20,7 @@ from billing.services import BillingError
 from billing.services.dues import record_payment, subscribe
 from club.models import Club, ClubMembership, ClubRole, Season
 from events.models import Attendance, Event
+from features.models import Maintenance
 from members.models import Member
 from shop.models import Order
 from teams.models import Position, StaffAssignment, Team, TeamMembership
@@ -1315,3 +1316,38 @@ class BillingFormRenderTests(ControlPanelTestBase):
         response = self.client.post(reverse("controlpanel:due_waive", args=[due.pk]), follow=True)
 
         self.assertContains(response, "remove them before waiving")
+
+
+class MaintenancePanelTests(ControlPanelTestBase):
+    def setUp(self):
+        super().setUp()
+        cache.clear()
+        self.addCleanup(cache.clear)
+
+    def test_closing_the_platform_records_who_did_it(self):
+        self.client.post(reverse("controlpanel:maintenance"), {"message": "Database upgrade."})
+
+        maintenance = Maintenance.current()
+        self.assertTrue(maintenance.is_active)
+        self.assertEqual(maintenance.message, "Database upgrade.")
+        self.assertEqual(maintenance.started_by, self.staff)
+
+    def test_posting_again_reopens_the_platform(self):
+        Maintenance.start(message="x", user=self.staff)
+
+        self.client.post(reverse("controlpanel:maintenance"), {})
+
+        self.assertFalse(Maintenance.is_on())
+
+    def test_every_panel_page_warns_while_the_platform_is_closed(self):
+        # Not a state to leave on by accident.
+        Maintenance.start(user=self.staff)
+
+        for url in (reverse("controlpanel:dashboard"), reverse("controlpanel:club_list"), reverse("controlpanel:features")):
+            self.assertContains(self.client.get(url), "closed for maintenance", msg_prefix=url)
+
+    def test_the_features_page_offers_the_switch(self):
+        response = self.client.get(reverse("controlpanel:features"))
+
+        self.assertContains(response, "Maintenance mode")
+        self.assertContains(response, "Close the platform")