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/middleware.py

@@ -0,0 +1,74 @@
+"""Platform lock-down.
+
+While maintenance is on, every club subdomain is closed and the base domain keeps only what
+is needed to *end* the maintenance: the control panel, the auth screens that get you into it,
+the static files those pages need, and the health check.
+
+The exemptions are the whole design. Close /accounts/ as well and you cannot sign in to turn
+maintenance off — a lock-down with no key, fixable only from a shell. Close /healthz and the
+load balancer concludes the node is dead and stops routing to it, which takes the control
+panel down with everything else.
+"""
+
+from django.http import JsonResponse
+from django.shortcuts import render
+from django.utils.translation import gettext_lazy as _
+
+from features.models import Maintenance
+
+#: Reachable on the base domain while the platform is locked down.
+OPEN_PREFIXES = (
+    "/controlpanel/",  # the point of the exercise
+    "/accounts/",  # ...which you cannot reach without signing in
+    "/admin/",
+    "/static/",
+    "/media/",
+    "/__reload__/",  # dev only; absent outside DEBUG
+)
+
+#: Reachable on every host, always. The health check must answer or the load balancer will
+#: take the node out of rotation and the control panel with it.
+ALWAYS_OPEN = ("/healthz",)
+
+RETRY_AFTER_SECONDS = 3600
+
+
+class MaintenanceMiddleware:
+    """Runs after ClubTenantMiddleware: whether a request is a club's or the platform's is
+    decided by ``request.club``, which the tenant middleware has just resolved."""
+
+    def __init__(self, get_response):
+        self.get_response = get_response
+
+    def __call__(self, request):
+        if not self.is_closed(request):
+            return self.get_response(request)
+
+        maintenance = Maintenance.current()
+        response = self.render(request, maintenance)
+        response["Retry-After"] = RETRY_AFTER_SECONDS
+
+        return response
+
+    def is_closed(self, request) -> bool:
+        if request.path.startswith(ALWAYS_OPEN):
+            return False
+
+        if not Maintenance.is_on():
+            return False
+
+        # A club subdomain is closed outright — no login, no shop, nothing.
+        if getattr(request, "club", None) is not None:
+            return True
+
+        # The base domain keeps the way back in.
+        return not request.path.startswith(OPEN_PREFIXES)
+
+    def render(self, request, maintenance):
+        message = maintenance.message or _("RosterChief is down for maintenance. It will be back shortly.")
+
+        # An API-ish caller gets JSON rather than a page of HTML it cannot read.
+        if request.headers.get("accept", "").startswith("application/json"):
+            return JsonResponse({"status": "maintenance", "detail": str(message)}, status=503)
+
+        return render(request, "maintenance.html", {"message": message, "maintenance": maintenance}, status=503)