fix(analytics): identify page views via fetch metadata, not router headers

Next.js strips the router-internal headers (RSC, next-router-prefetch)
before the request proxy runs, so prefetch detection based on them never
fired: link prefetches were counted as page views. A page view is now a
browser navigation, identified by the standard sec-fetch-dest: document
header, which survives into the proxy. Prefetch payloads, subresource
fetches and metadata-less clients are skipped as non-navigation
requests.

apps/web/proxy.ts

@@ -3,6 +3,10 @@
  * the pure domain decision, and records a page view without touching the
  * response. All analytics happen server-side; no tracking script ships to
  * the browser.
+ *
+ * Page views are browser navigations (`sec-fetch-dest: document`).
+ * Router-internal headers (RSC, next-router-prefetch) are stripped by the
+ * server before this shell runs and cannot be used here.
  */
 
 import { NextResponse, type NextRequest } from "next/server";
@@ -18,14 +22,10 @@ const firstForwardedIp = (forwardedFor: string | null): string =>
 
 const hostnameWithoutPort = (host: string): string => host.replace(/:\d+$/, "");
 
-const isPrefetchRequest = (request: NextRequest): boolean =>
-  request.headers.get("next-router-prefetch") === "1" ||
-  request.headers.get("purpose") === "prefetch" ||
-  request.headers.get("sec-purpose") === "prefetch";
-
 export default function proxy(request: NextRequest): NextResponse {
   const decision = decidePageview({
     method: request.method,
+    secFetchDest: request.headers.get("sec-fetch-dest") ?? "",
     pathname: request.nextUrl.pathname,
     search: request.nextUrl.search,
     hostname: hostnameWithoutPort(
@@ -37,8 +37,6 @@ export default function proxy(request: NextRequest): NextResponse {
     language: primaryLanguage(request.headers.get("accept-language")),
     userAgent: request.headers.get("user-agent") ?? "",
     clientIp: firstForwardedIp(request.headers.get("x-forwarded-for")),
-    isRscNavigation: request.headers.get("rsc") === "1",
-    isPrefetch: isPrefetchRequest(request),
   });
 
   if (decision.track) {