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/index.ts

@@ -109,7 +109,6 @@ const atom = new Hono();
 const entries = new Hono();
 const files = new Hono();
 const admin = new Hono();
-const hub = new Hono();
 
 // Webhook security middleware for /inbound - verify ForwardEmail.net IP
 api.use("/inbound", async (c, next) => {
@@ -151,9 +150,6 @@ files.get("/:attachmentId/:filename", handleFiles);
 // Admin routes (protected)
 admin.route("/", handleAdmin);
 
-// Hub (WebSub) routes
-hub.route("/", hubRouter);
-
 // Mount the route groups
 app.route("/api", api);
 app.route("/rss", rss);
@@ -161,7 +157,7 @@ app.route("/atom", atom);
 app.route("/entries", entries);
 app.route("/files", files);
 app.route("/admin", admin);
-app.route("/hub", hub);
+app.route("/hub", hubRouter);
 
 // Root path redirects to admin dashboard
 app.get("/", (c) => c.redirect("/admin"));