Two optional settings for deployments behind a CDN such as Cloudflare.
CLIENT_IP_HEADER names the header that holds the visitor's real IP
(cf-connecting-ip on Cloudflare). The per-IP daily quota used the first
X-Forwarded-For entry, which visitors can set to anything, so a made-up
IP on every request got a fresh quota.
ORIGIN_SECRET makes proxy.ts refuse /api requests whose X-Origin-Secret
header does not match. The CDN adds the header, so a call that skips the
CDN, and could fake the IP header, gets 403. Pages are not checked, which
keeps health checks on / working.
Both are unset by default, and nothing changes then.