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/commands.py

@@ -0,0 +1,31 @@
+"""Scheduled work refuses to run while the platform is locked down.
+
+Deliberately opt-in, per command, rather than a blanket guard on BaseCommand: maintenance is
+usually declared IN ORDER to run `migrate` or `collectstatic`, and a guard that blocked those
+would make the mode useless — you would have to turn it off to do the work you turned it on
+for. Only the domain jobs (which write club data, archive clubs, or import members) stand
+down.
+"""
+
+from django.core.management.base import BaseCommand, CommandError
+
+from features.models import Maintenance
+
+
+class MaintenanceAwareCommand(BaseCommand):
+    """A command that must not run while the platform is closed."""
+
+    def execute(self, *args, **options):
+        if Maintenance.is_on() and not options.get("ignore_maintenance"):
+            raise CommandError("The platform is in maintenance mode; this command stands down. Pass --ignore-maintenance to override.")
+
+        return super().execute(*args, **options)
+
+    def create_parser(self, prog_name, subcommand, **kwargs):
+        parser = super().create_parser(prog_name, subcommand, **kwargs)
+        parser.add_argument(
+            "--ignore-maintenance",
+            action="store_true",
+            help="Run even though the platform is in maintenance mode.",
+        )
+        return parser