Implemented phase 5 for the POC prototype in the Website service.

Services/Website/Tests/App/AppTests.swift

@@ -173,6 +173,38 @@ struct AppTests {
         }
     }
 
+    @Test
+    func `landing page to catch up once a copy is sold`() async throws {
+        try await app(
+            staticFilesPath: staticFilesPath
+        ).test(.router) { client in
+            let sold = #"href="/listing/echo-country-3""#
+            let before = try await client.execute(uri: "/", method: .get) { response in
+                #expect(String(buffer: response.body).contains(sold))
+
+                return try #require(response.headers[.eTag])
+            }
+            let form: HTTPFields = [.contentType: "application/x-www-form-urlencoded"]
+            let cookie = try await client.execute(uri: "/sign-in", method: .post, headers: form, body: ByteBuffer(string: "return=/")) { response in
+                String(try #require(response.headers[.setCookie]).prefix { $0 != ";" })
+            }
+            var signedIn = form
+
+            signedIn[.cookie] = cookie
+
+            try await client.execute(uri: "/listing/echo-country-3/buy", method: .post, headers: signedIn) { response in
+                #expect(response.status == .seeOther)
+            }
+
+            try await client.execute(uri: "/", method: .get, headers: [.ifNoneMatch: before]) { response in
+                // The page changed, so the entity tag a browser holds no longer revalidates.
+                #expect(response.status == .ok)
+                #expect(response.headers[.eTag] != before)
+                #expect(!String(buffer: response.body).contains(sold))
+            }
+        }
+    }
+
     @Test
     func `error page to be served for an unknown listing`() async throws {
         try await app(