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/src/domain/pageview-decision.spec.ts

@@ -8,6 +8,7 @@ const humanRequest = (
   overrides: Partial<IncomingPageviewRequest> = {},
 ): IncomingPageviewRequest => ({
   method: "GET",
+  secFetchDest: "document",
   pathname: "/",
   search: "",
   hostname: "mintel.me",
@@ -15,13 +16,11 @@ const humanRequest = (
   language: "de-DE",
   userAgent: "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) Chrome/131.0.0.0",
   clientIp: "84.117.2.55",
-  isRscNavigation: false,
-  isPrefetch: false,
   ...overrides,
 });
 
 describe("decidePageview", () => {
-  it("tracks a document request for the homepage", () => {
+  it("tracks a browser navigation for the homepage", () => {
     const decision = decidePageview(humanRequest());
     expect(decision).toEqual({
       track: true,
@@ -60,11 +59,6 @@ describe("decidePageview", () => {
     expect(decision).toMatchObject({ track: true });
   });
 
-  it("tracks a client-side route change (RSC navigation without prefetch)", () => {
-    const decision = decidePageview(humanRequest({ isRscNavigation: true }));
-    expect(decision).toMatchObject({ track: true });
-  });
-
   it("accepts the method in lower case", () => {
     const decision = decidePageview(humanRequest({ method: "get" }));
     expect(decision).toMatchObject({ track: true });
@@ -79,16 +73,17 @@ describe("decidePageview", () => {
     expect(decision).toEqual({ track: false, reason: "non-get-method" });
   });
 
-  it("skips a router prefetch", () => {
-    const decision = decidePageview(humanRequest({ isPrefetch: true }));
-    expect(decision).toEqual({ track: false, reason: "prefetch" });
-  });
-
-  it("skips a prefetch even when the agent is a bot", () => {
-    const decision = decidePageview(
-      humanRequest({ isPrefetch: true, userAgent: "curl/8.1.2" }),
-    );
-    expect(decision).toEqual({ track: false, reason: "prefetch" });
+  it.each([
+    ["a router prefetch payload", "empty"],
+    ["a script subresource", "script"],
+    ["an image subresource", "image"],
+    ["a request without fetch metadata", ""],
+  ])("skips %s", (_label, secFetchDest) => {
+    const decision = decidePageview(humanRequest({ secFetchDest }));
+    expect(decision).toEqual({
+      track: false,
+      reason: "non-navigation-request",
+    });
   });
 
   it.each([
@@ -135,4 +130,11 @@ describe("decidePageview", () => {
     const decision = decidePageview(humanRequest({ userAgent }));
     expect(decision).toEqual({ track: false, reason: "bot-user-agent" });
   });
+
+  it("skips a bot even when its request looks like a navigation", () => {
+    const decision = decidePageview(
+      humanRequest({ userAgent: "curl/8.1.2", secFetchDest: "document" }),
+    );
+    expect(decision).toEqual({ track: false, reason: "bot-user-agent" });
+  });
 });