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) {