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>

api/relay.ts

@@ -1,8 +1,9 @@
 /**
- * Vercel-compatible relay handler.
+ * Vercel-compatible relay handler (Bun runtime).
  *
  * This file is the entry point for Vercel serverless function deployments.
- * It exports a `default` fetch handler that Vercel invokes per-request.
+ * It exports `{ fetch }` — the contract Vercel's Bun runtime expects for
+ * serverless functions.
  *
  * It reuses the same relay logic from `src/lib/` and `src/middleware/` as
  * the standalone Bun.serve() server, but:
@@ -348,13 +349,17 @@ async function handleRelay(req: Request): Promise<Response> {
 // ─── Exported Vercel Function Handler ───────────────────────────────────────────
 
 /**
- * Vercel serverless function handler.
+ * Vercel Bun runtime handler.
  *
- * Invoked per-request by Vercel's infrastructure. Handles routing,
- * middleware, and relay logic — same semantics as the standalone
- * Bun.serve() server, minus WebSocket support.
+ * Vercel's Bun runtime expects a `default` export that is an object with
+ * a `fetch` method — NOT a bare default function.  This matches the
+ * standard `Bun.serve()` handler shape.
+ *
+ * Handles routing, middleware, and relay logic — same semantics as the
+ * standalone Bun.serve() server, minus WebSocket support.
  */
-export default async function handler(req: Request): Promise<Response> {
+export default {
+	async fetch(req: Request): Promise<Response> {
 		const url = new URL(req.url);
 
 		// Static routes
@@ -385,4 +390,5 @@ export default async function handler(req: Request): Promise<Response> {
 
 		// Generic HTTP relay
 		return handleRelay(req);
-}
+	},
+};

vercel.json

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