Add scheduled cleanup for abandoned setup flows

.env.example

@@ -1,5 +1,4 @@
 DDNS_ADMIN_PASSWORD=change-me-long-random-admin-password
-DDNS_SESSION_SECRET=change-me-long-random-session-secret
 DDNS_PUBLIC_BASE_URL=https://ddns.example.net
 DDNS_HOSTNAME_SUFFIX=ddns.example.net
 DDNS_DATABASE=/data/ddns.sqlite3
@@ -11,4 +10,6 @@ DDNS_CLOUDFLARE_ZONE_ID=change-me-cloudflare-zone-id
 DDNS_TTL=60
 DDNS_RATE_LIMIT_PER_MINUTE=60
 DDNS_CLEANUP_CHALLENGE_HOURS=72
+DDNS_CLEANUP_UNUSED_ACCOUNT_HOURS=168
+DDNS_CLEANUP_INTERVAL_SECONDS=3600
 DDNS_TRUSTED_PROXY_IPS=127.0.0.1,::1