refactor(cors): replace manual CORS middleware with hono/cors

Fixes a bug where routes returning raw `new Response()` (RSS, Atom,
entries) were not receiving CORS headers — hono/cors applies headers
after next(), covering all response paths.

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

package.json

@@ -45,6 +45,7 @@
     "wrangler": "^4.63.0"
   },
   "dependencies": {
+    "@hono/zod-validator": "^0.8.0",
     "feed": "^5.2.0",
     "hono": "^4.11.7",
     "postal-mime": "^2.7.4",