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/templates/controlpanel/base.html

@@ -6,6 +6,16 @@
 {% endblock title %}
 
 {% block main %}
+    {% if maintenance_on %}
+        <div class="alert alert-error mb-6">
+            {% lucide "wrench" size=20 %}
+            <span>
+                <strong>The platform is closed for maintenance.</strong>
+                Clubs see a maintenance page and the scheduled jobs are standing down.
+            </span>
+            <a class="btn btn-sm" href="{% url 'controlpanel:features' %}">Reopen</a>
+        </div>
+    {% endif %}
     <div class="mb-6 flex flex-wrap items-center justify-between gap-3">
         <div class="flex flex-col gap-2">
             <h1 class="text-3xl font-bold">

controlpanel/templates/controlpanel/features.html

@@ -1,5 +1,5 @@
 {% extends "controlpanel/base.html" %}
-{% load lucide %}
+{% load lucide ui %}
 
 {% block heading %}Features{% endblock heading %}
 
@@ -8,6 +8,48 @@
 {% endblock actions %}
 
 {% block panel %}
+    {% comment %}
+        The lock-down. Clubs get a maintenance page, the scheduled jobs stand down, and the
+        control panel and the auth screens stay open — otherwise you could not sign in to
+        turn it back off.
+    {% endcomment %}
+    <div class="card mb-6 bg-base-100 shadow {% if maintenance.is_active %}border-l-4 border-error{% endif %}">
+        <div class="card-body">
+            <div class="flex flex-wrap items-start justify-between gap-4">
+                <div>
+                    <h2 class="card-title text-base">{% lucide "wrench" size=18 %} Maintenance mode</h2>
+                    {% if maintenance.is_active %}
+                        <p class="text-sm">
+                            <span class="badge badge-error gap-1">{% lucide "lock" size=12 %} Platform closed</span>
+                            since {{ maintenance.started_at|date:"j M Y, H:i" }}{% if maintenance.started_by %} by {{ maintenance.started_by.email }}{% endif %}.
+                        </p>
+                        {% if maintenance.message %}<p class="mt-1 text-sm opacity-70">“{{ maintenance.message }}”</p>{% endif %}
+                    {% else %}
+                        <p class="text-sm opacity-70">
+                            Closes every club subdomain and stands the scheduled jobs down. The control panel and the sign-in screens stay open.
+                        </p>
+                    {% endif %}
+                </div>
+            </div>
+
+            <form class="mt-2" method="post" action="{% url 'controlpanel:maintenance' %}">
+                {% csrf_token %}
+                {% if not maintenance.is_active %}
+                    <div class="form-control my-2 w-full max-w-xl">
+                        <label class="label" for="{{ maintenance_form.message.id_for_label }}">
+                            <span class="label-text">{{ maintenance_form.message.label }}</span>
+                        </label>
+                        {{ maintenance_form.message|daisy }}
+                        <span class="label-text-alt mt-1 block text-base-content/70">{{ maintenance_form.message.help_text }}</span>
+                    </div>
+                    <button class="btn btn-error gap-2" type="submit">{% lucide "lock" size=16 %} Close the platform</button>
+                {% else %}
+                    <button class="btn btn-success gap-2" type="submit">{% lucide "lock-open" size=16 %} Reopen the platform</button>
+                {% endif %}
+            </form>
+        </div>
+    </div>
+
     <div class="card mb-6 bg-base-100 shadow">
         <div class="card-body">
             <h2 class="card-title text-base">{% lucide "flag" size=18 %} Flags</h2>