fix: correct Vercel Bun runtime config and handler export format

Per Vercel docs (vercel.com/docs/functions/runtimes/bun):
- Use top-level "bunVersion": "1.x" in vercel.json (NOT functions block)
- Export default { async fetch(request) { ... } } — object with fetch method
  (not a bare default function)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

vercel.json

@@ -1,9 +1,5 @@
 {
-  "functions": {
-    "api/relay.ts": {
-      "runtime": "@vercel/node@5"
-    }
-  },
+  "bunVersion": "1.x",
   "rewrites": [
     { "source": "/(.*)", "destination": "/api/relay" }
   ]