fix(feeds): self link uses configured domain, not request host

The RSS/Atom/JSON self link was derived from the request origin, leaking
the workers.dev host when reached directly instead of via the custom
domain. Use the configured-domain URL builders so self matches alternate.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/routes/rss.test.ts

@@ -50,7 +50,9 @@ describe("RSS Feed Route", () => {
       const res = await testApp.request("/empty-feed", {}, mockEnv);
       const link = res.headers.get("Link") ?? "";
       expect(link).toContain(`rel="hub"`);
-      expect(link).toContain(`rel="self"`);
+      expect(link).toContain(
+        `<https://${mockEnv.DOMAIN}/rss/empty-feed>; rel="self"`,
+      );
     });
   });