fix: don't read Cloudflare's injected jsd script as a refusal

demonicscans.org turned JS detections on, so Cloudflare began injecting
/cdn-cgi/challenge-platform/scripts/jsd/main.js into ordinary 200 pages.
isInterstitial matched the bare /cdn-cgi/challenge-platform/ prefix, so
every real demonic series page became errChallengeHeld: two per pass
parked the Lane in the 15m refusal backoff while plain TLS was in fact
returning the full page (200, 133KB, 22 chapter anchors, verified live
2026-08-16).

Match the challenge orchestration path /cdn-cgi/challenge-platform/h/
instead — served only by the interstitial itself, stable across its
wording and locale.

backend/internal/latest/browser_test.go

@@ -117,6 +117,25 @@ func TestBrowserOnlyCoverURL(t *testing.T) {
 		})
 	}
 }
+
+// The jsd script is injected into ordinary 200 pages when a zone turns JS
+// detections on; only the orchestration path means the page itself is the
+// challenge. Conflating the two parked the demonic Lane in refusal backoff
+// while every fetch was in fact the real series page (observed 2026-08-16).
+func TestIsInterstitial(t *testing.T) {
+	if !isInterstitial(challengeFixture) {
+		t.Fatal("challenge page not detected as interstitial")
+	}
+	const jsdInjected = `<html><head><title>The Possessed Grappler</title>
+<script src="/cdn-cgi/challenge-platform/scripts/jsd/main.js"></script></head>
+<body><a  href="/chaptered.php?manga=13721&chapter=22">Chapter 22</a></body></html>`
+	if isInterstitial(jsdInjected) {
+		t.Fatal("real page carrying the injected jsd script misread as interstitial")
+	}
+	if got, ok := demonicLatestChapter("", jsdInjected); !ok || got.Label != "Chapter 22" {
+		t.Fatalf("demonicLatestChapter = %+v, ok = %v, want Chapter 22", got, ok)
+	}
+}
 func TestClassifyBrowserInterruption(t *testing.T) {
 	if err := classifyBrowserError(context.Background(), true, context.Canceled); !errors.Is(err, errBrowserInterrupted) {
 		t.Fatalf("classifyBrowserError(context.Canceled) = %v, want browser interruption", err)