Express.js backend returns 404 on Vercel deployment

I'm trying to deploy an Express.js + MongoDB + Mongoose backend to Vercel, but I’m getting this error when accessing any route (including /api/v1):

404: NOT_FOUND  
Code: NOT_FOUND

Project structure:

Backend/
├── src/
│   ├── app.js
│   ├── index.js
│   ├── routers/
│   │   ├── user.route.js
│   │   ├── company.route.js
│   │   └── ... (other routes)
├── vercel.json
├── package.json
└── ...

vercel.json:

{
  "version": 2,
  "builds": [
    {
      "src": "src/index.js",
      "use": "@vercel/node"
    }
  ],
  "routes": [
    {
      "src": "/(.*)",
      "dest": "src/index.js"
    }
  ]
}

app.js:

import express from "express";
import cookieParser from "cookie-parser";
import cors from "cors";
import dotenv from "dotenv";
import indexRoutes from "./routers/index.routes.js";
import userRoutes from "./routers/user.route.js";  
import companyRoute from "./routers/company.route.js";
import jobRoute from "./routers/job.route.js";
import applicationRoute from "./routers/application.route.js";
import adminRoute from "./routers/admin.route.js";
import interviewRoute from "./routers/interview.route.js";
import portfolioRoute from "./routers/portfolio.route.js";

dotenv.config();

const app = express();

// Middleware
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(cookieParser());

const corsOptions = {
    origin: "http://localhost:5173",
    credentials: true,
    methods: ["GET", "POST", "PUT", "DELETE"],
    allowedHeaders: ["Content-Type", "Authorization"],
};
app.use(cors(corsOptions));

// Test Route
app.get("/", (req, res) => {
    return res.status(200).json({
        message: "Connected to the server",
        success: true
    });
});

// Routes
app.use("/api/v1/user", userRoutes);
app.use("/api/v1/company", companyRoute);
app.use("/api/v1/job", jobRoute);
app.use("/api/v1/application", applicationRoute);
app.use("/api/v1/admin", adminRoute);
app.use("/api/v1/interviews", interviewRoute);
app.use("/api/v1/portfolio", portfolioRoute);

export default app;

When deployed to Vercel:

• Accessing https://smartjobportal-server.vercel.app/api/v1 (or any route) gives 404 NOT_FOUND.
• Even root / route returns 404.
• Logs show no output, no crash, nothing.

What I’ve Tried:

1. Confirmed all routes are properly defined.
2. MongoDB connection works locally.
3. app.listen() works when running npm run dev or node src/index.js.
4. Added a test / route to verify.
5. Deployed with Vercel CLI (successful build and deploy).
6. vercel.json correctly configured to point to src/index.js.

How can I fix this 404 error and get my Express routes working on Vercel?