Fix: Include source and dist directories in Vercel serverless functions

Vercel serverless functions are bundled separately and don't automatically
include project directories. Added includeFiles config to ensure source/
and dist/ are available to API handlers at runtime.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

vercel.json

@@ -1,6 +1,11 @@
 {
   "$schema": "https://openapi.vercel.sh/vercel.json",
   "buildCommand": "bun run build",
-  "outputDirectory": "build"
+  "outputDirectory": "build",
+  "functions": {
+    "api/**/*.js": {
+      "includeFiles": "source/**,dist/**"
+    }
+  }
 }