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>

compose.behind-proxy.yaml

@@ -0,0 +1,55 @@
+# A dev/test deployment on a server that ALREADY runs Caddy on :80/:443.
+#
+#   docker compose -f compose.behind-proxy.yaml up -d
+#
+# The difference from compose.yaml is only what listens on the network: no caddy service, and
+# web publishes on the loopback instead of the public interface. The host's Caddy reverse
+# proxies to it (see DEPLOYMENT.md, "Behind an existing Caddy").
+#
+# Publishing on 127.0.0.1 and not 0.0.0.0 is the point: bound to all interfaces, a test
+# instance is reachable on http://<server-ip>:8001 with no TLS, bypassing the proxy and every
+# security header with it.
+
+name: rosterchief-test
+
+services:
+  web:
+    build: .
+    restart: unless-stopped
+    env_file: .env.production
+    ports:
+      - "127.0.0.1:${WEB_PORT:-8001}:8000"
+    depends_on:
+      db:
+        condition: service_healthy
+      redis:
+        condition: service_started
+    healthcheck:
+      test: ["CMD", "curl", "-fsS", "http://localhost:8000/healthz"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+      start_period: 20s
+
+  db:
+    image: postgres:17-alpine
+    restart: unless-stopped
+    environment:
+      POSTGRES_DB: ${POSTGRES_DB:-rosterchief}
+      POSTGRES_USER: ${POSTGRES_USER:-rosterchief}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:?set a database password}
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-rosterchief}"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
+  redis:
+    image: redis:7-alpine
+    restart: unless-stopped
+    command: ["redis-server", "--save", "", "--appendonly", "no"]
+
+volumes:
+  pgdata: