fix: add Vercel-compatible relay handler and fix vercel.json runtime

- Create api/relay.ts: Vercel serverless function handler that reuses
  relay logic from src/lib/relay-utils.ts and src/middleware/
- Export default fetch handler (no Bun.serve()) for Vercel Functions
- Fix vercel.json: use @vercel/node@5 runtime with proper routing via
  rewrites to catch all traffic to the relay handler
- Remove stale buildCommand/outputDirectory (Vercel handles function builds)

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

vercel.json

@@ -1,9 +1,10 @@
 {
-  "buildCommand": "bun install",
-  "outputDirectory": "dist",
   "functions": {
-    "src/index.ts": {
-      "runtime": "bun@1"
+    "api/relay.ts": {
+      "runtime": "@vercel/node@5"
     }
-  }
+  },
+  "rewrites": [
+    { "source": "/(.*)", "destination": "/api/relay" }
+  ]
 }