Add forwarder_hosts config to scope upstream forwarder routing per host

deploy/cloudflare-worker/worker.js

@@ -38,7 +38,9 @@ export default {
             }
 
             const upstreamUrl = (env && env.UPSTREAM_FORWARDER_URL) || "";
-            if (upstreamUrl) {
+            // f === 1: forward; f === 0: skip; missing: legacy client → forward (compat).
+            const wantForward = (req.f === 1) || (req.f === undefined);
+            if (upstreamUrl && wantForward) {
                 const upstreamResp = await forwardViaUpstream(req, env, upstreamUrl);
                 if (upstreamResp) return upstreamResp;
                 // fall through to direct fetch only when fail-mode is open

deploy/gas/Code.gs

@@ -105,7 +105,7 @@ function _buildWorkerPayload(req) {
     }
   }
 
-  return {
+  var out = {
     u: req.u,
     m: (req.m || "GET").toUpperCase(),
     h: headers,
@@ -113,6 +113,8 @@ function _buildWorkerPayload(req) {
     ct: req.ct || null,
     r: req.r !== false
   };
+  if (typeof req.f === "number") out.f = req.f;
+  return out;
 }
 
 function doGet(e) {