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/src/auth/auth.service.ts

@@ -1,5 +1,7 @@
 import { Injectable, Logger } from "@nestjs/common";
 import type { PrismaClient } from "@prisma/client";
+import type { IncomingMessage, ServerResponse } from "http";
+import { toNodeHandler } from "better-auth/node";
 import { PrismaService } from "../prisma/prisma.service";
 import { createAuth, type Auth } from "./auth.config";
 
@@ -7,11 +9,13 @@ import { createAuth, type Auth } from "./auth.config";
 export class AuthService {
   private readonly logger = new Logger(AuthService.name);
   private readonly auth: Auth;
+  private readonly nodeHandler: (req: IncomingMessage, res: ServerResponse) => Promise<void>;
 
   constructor(private readonly prisma: PrismaService) {
     // PrismaService extends PrismaClient and is compatible with BetterAuth's adapter
     // Cast is safe as PrismaService provides all required PrismaClient methods
     this.auth = createAuth(this.prisma as unknown as PrismaClient);
+    this.nodeHandler = toNodeHandler(this.auth);
   }
 
   /**
@@ -21,6 +25,14 @@ export class AuthService {
     return this.auth;
   }
 
+  /**
+   * Get Node.js-compatible request handler for BetterAuth.
+   * Wraps BetterAuth's Web API handler to work with Express/Node.js req/res.
+   */
+  getNodeHandler(): (req: IncomingMessage, res: ServerResponse) => Promise<void> {
+    return this.nodeHandler;
+  }
+
   /**
    * Get user by ID
    */