lith: auto-purge Cloudflare cache on system/public/* changes
Webhook now POSTs purge_everything via a zone-scoped CF API token
whenever a deploy touches any static file under system/public/*.
Env: CLOUDFLARE_PURGE_TOKEN, CLOUDFLARE_ZONE_ID (in /opt/ac/system/.env).
Non-fatal on API failure — deploy continues.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>