fix(websub): add missing WebSub Link header to Atom feed

The RSS feed already advertised hub and self via the Link response
header, but the Atom feed was missing it entirely. Subscribers using
Atom had no way to discover the hub for real-time push notifications.

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

src/routes/atom.ts

@@ -51,11 +51,17 @@ export async function handle(c: Context): Promise<Response> {
     const baseUrl = `https://${env.DOMAIN}`;
     const atomXml = generateAtomFeed(feedConfig, emailsData, baseUrl, feedId);
 
+    const linkHeader = [
+      `<${baseUrl}/hub>; rel="hub"`,
+      `<${baseUrl}/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) {