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>

features/migrations/0002_maintenance.py

@@ -0,0 +1,33 @@
+# Generated by Django 6.0.6 on 2026-07-14 08:00
+
+import django.db.models.deletion
+import uuid
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('features', '0001_initial'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Maintenance',
+            fields=[
+                ('created', models.DateTimeField(auto_now_add=True, verbose_name='created')),
+                ('modified', models.DateTimeField(auto_now=True, verbose_name='modified')),
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('is_active', models.BooleanField(default=False, verbose_name='active')),
+                ('message', models.TextField(blank=True, help_text='Shown to clubs while the platform is locked down.', verbose_name='message')),
+                ('started_at', models.DateTimeField(blank=True, null=True, verbose_name='started at')),
+                ('started_by', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='maintenance_windows', to=settings.AUTH_USER_MODEL, verbose_name='started by')),
+            ],
+            options={
+                'verbose_name': 'maintenance',
+                'verbose_name_plural': 'maintenance',
+            },
+        ),
+    ]