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>

templates/maintenance.html

@@ -0,0 +1,24 @@
+{% extends base_template %}
+{% load lucide %}
+
+{% comment %}
+    Rendered through the tenant's own skin, so a club sees its own logo and colours rather
+    than a bare error page — this is the platform being unavailable, not the club being gone.
+{% endcomment %}
+
+{% block head_title %}Maintenance{% endblock head_title %}
+
+{% block main %}
+    <div class="flex justify-center">
+        <div class="card w-full max-w-xl bg-base-100 shadow">
+            <div class="card-body items-center text-center">
+                <div class="text-warning">{% lucide "wrench" size=48 %}</div>
+                <h1 class="card-title mt-2">Down for maintenance</h1>
+                <p class="opacity-80">{{ message }}</p>
+                {% if maintenance.started_at %}
+                    <p class="text-xs opacity-60">Since {{ maintenance.started_at|date:"j M Y, H:i" }}</p>
+                {% endif %}
+            </div>
+        </div>
+    </div>
+{% endblock main %}