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/forms.py

@@ -103,3 +103,15 @@ class OpenPeriodForm(forms.Form):
     """Renew, or reactivate an archived club."""
 
     start = forms.DateField(required=False, widget=forms.DateInput(attrs={"type": "date"}), label=_("Period starts"), help_text=_("Left blank, it continues from the end of the last period — so a lapsed year is still owed."))
+
+
+class MaintenanceForm(forms.Form):
+    """Closing the platform is a deliberate act, so it takes a sentence explaining itself —
+    that message is the only thing a club will see."""
+
+    message = forms.CharField(
+        required=False,
+        widget=forms.Textarea(attrs={"rows": 2}),
+        label=_("Message"),
+        help_text=_("Shown to every club while the platform is closed. Left blank, they get a generic notice."),
+    )