fix(#410): use toNodeHandler for BetterAuth Express compatibility

BetterAuth expects Web API Request objects (Fetch API standard) with
headers.get(), but NestJS/Express passes IncomingMessage objects with
headers[] property access. Use better-auth/node's toNodeHandler to
properly convert between Express req/res and BetterAuth's Web API handler.

Also fixes vitest SWC config to read the correct tsconfig for NestJS
decorator metadata emission, which was causing DI injection failures
in tests.

Fixes #410

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

apps/api/vitest.config.ts

@@ -26,7 +26,7 @@ export default defineConfig({
   },
   plugins: [
     swc.vite({
-      module: { type: "es6" },
+      tsconfigFile: path.resolve(__dirname, "tsconfig.json"),
     }),
   ],
 });