fix(websub): require feed existence for subscriptions, remove atom hub header, simplify router mounting

- Add KV feed existence check in hub.ts to prevent SSRF via non-existent feeds (returns 404)
- Treat empty string hub.secret as absent (|| instead of ??)
- Remove misleading hub Link header from atom.ts (hub only supports RSS topics)
- Simplify double-layered hub router in index.ts (direct app.route instead of nested Hono)
- Update hub.test.ts to seed KV with feed config for tests requiring valid subscribe/unsubscribe

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/atom.ts

@@ -51,17 +51,11 @@ export async function handle(c: Context): Promise<Response> {
     const baseUrl = `https://${env.DOMAIN}`;
     const atomXml = generateAtomFeed(feedConfig, emailsData, baseUrl, feedId);
 
-    const linkHeader = [
-      `<https://${env.DOMAIN}/hub>; rel="hub"`,
-      `<https://${env.DOMAIN}/atom/${feedId}>; rel="self"`,
-    ].join(", ");
-
     return new Response(atomXml, {
       status: 200,
       headers: {
         "Content-Type": "application/atom+xml",
         "Cache-Control": "max-age=1800",
-        Link: linkHeader,
       },
     });
   } catch (error) {

src/routes/hub.test.ts

@@ -127,6 +127,10 @@ describe("POST /hub — input validation", () => {
   it("returns 400 when hub.secret exceeds 200 bytes", async () => {
     const app = makeApp();
     const env = createMockEnv();
+    await env.EMAIL_STORAGE.put(
+      "feed:feed1:config",
+      JSON.stringify({ title: "Feed 1" }),
+    );
     const res = await app.request(
       "/hub",
       hubBody({
@@ -145,6 +149,10 @@ describe("POST /hub — subscribe", () => {
   it("returns 202 for valid subscribe request", async () => {
     const app = makeApp();
     const env = createMockEnv();
+    await env.EMAIL_STORAGE.put(
+      "feed:feed1:config",
+      JSON.stringify({ title: "Feed 1" }),
+    );
     server.use(
       http.get("https://cb.example/sub", ({ request }) => {
         const challenge =
@@ -167,6 +175,10 @@ describe("POST /hub — subscribe", () => {
   it("accepts hub.lease_seconds within range", async () => {
     const app = makeApp();
     const env = createMockEnv();
+    await env.EMAIL_STORAGE.put(
+      "feed:feed1:config",
+      JSON.stringify({ title: "Feed 1" }),
+    );
     server.use(
       http.get("https://cb.example/sub", ({ request }) => {
         const challenge =
@@ -186,12 +198,31 @@ describe("POST /hub — subscribe", () => {
     );
     expect(res.status).toBe(202);
   });
+
+  it("returns 404 when feed does not exist", async () => {
+    const app = makeApp();
+    const env = createMockEnv();
+    const res = await app.request(
+      "/hub",
+      hubBody({
+        "hub.mode": "subscribe",
+        "hub.topic": `https://${env.DOMAIN}/rss/nonexistent`,
+        "hub.callback": "https://cb.example/sub",
+      }),
+      env,
+    );
+    expect(res.status).toBe(404);
+  });
 });
 
 describe("POST /hub — unsubscribe", () => {
   it("returns 202 for valid unsubscribe request", async () => {
     const app = makeApp();
     const env = createMockEnv();
+    await env.EMAIL_STORAGE.put(
+      "feed:feed1:config",
+      JSON.stringify({ title: "Feed 1" }),
+    );
     server.use(
       http.get("https://cb.example/sub", ({ request }) => {
         const challenge =

src/routes/hub.ts

@@ -80,7 +80,16 @@ hubRouter.post("/", async (c) => {
   }
   const feedId = match[1];
 
-  const secret = form.get("hub.secret") ?? undefined;
+  // Verify the feed exists before accepting any subscription
+  const feedConfig = await env.EMAIL_STORAGE.get(
+    `feed:${feedId}:config`,
+    "json",
+  );
+  if (!feedConfig) {
+    return c.text("Not Found: feed does not exist", 404);
+  }
+
+  const secret = form.get("hub.secret") || undefined; // "" → undefined
   if (secret && secret.length > 200) {
     return c.text("Bad Request: hub.secret must be under 200 bytes", 400);
   }