feat(api): count the quota by the CDN's client IP and refuse direct calls

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.

lib/user-id.ts

@@ -2,10 +2,19 @@
  * Generate a userId from request for tracking purposes.
  * Uses base64url encoding of IP for URL-safe identifier.
  * Note: base64 is reversible - this is NOT privacy protection.
+ *
+ * The first X-Forwarded-For entry is whatever the visitor sent, so behind a
+ * CDN set CLIENT_IP_HEADER to the header it fills with the real IP (e.g.
+ * cf-connecting-ip), and ORIGIN_SECRET so requests that skip the CDN are
+ * refused (see proxy.ts).
  */
 export function getUserIdFromRequest(req: Request): string {
-    const forwardedFor = req.headers.get("x-forwarded-for")
-    const rawIp = forwardedFor?.split(",")[0]?.trim() || "anonymous"
+    const ipHeader = process.env.CLIENT_IP_HEADER
+    const rawIp =
+        (ipHeader
+            ? req.headers.get(ipHeader)?.trim()
+            : req.headers.get("x-forwarded-for")?.split(",")[0]?.trim()) ||
+        "anonymous"
     return rawIp === "anonymous"
         ? rawIp
         : `user-${Buffer.from(rawIp).toString("base64url")}`