```javascript
const express = require('express');
const bodyParser = require('body-parser');
const cors = require('cors');
const app = express();

// Middleware
app.use(cors());
app.use(bodyParser.json());
app.use(express.static('public'));

// Mock database for subscriptions
let subscriptions = [];

// API Routes
app.post('/api/subscribe', (req, res) => {
  const { email } = req.body;
  
  if (!email) {
    return res.status(400).json({ error: 'Email is required' });
  }

  // Check if email already exists
  if (subscriptions.includes(email)) {
    return res.status(400).json({ error: 'Email already subscribed' });
  }

  // Add to subscriptions
  subscriptions.push(email);
  console.log('New subscription:', email);
  
  res.status(200).json({ message: 'Subscription successful' });
});

// Start server
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});
```