feat: add Next.js metrics dashboard for real-time visualization

Add a lightweight Next.js dashboard to visualize OpenProxy metrics in real-time. The dashboard provides comprehensive insights into LLM API usage, costs, and performance.

Features:
- Real-time metrics overview (requests, tokens, costs, response times)
- Model breakdown with usage statistics
- Hourly trends visualization with charts
- Recent requests table with detailed information
- Auto-refresh every 30 seconds
- Configurable time ranges (1h, 6h, 24h, 7d)

Technical details:
- Built with Next.js 14 and React 18
- Uses Recharts for data visualization
- Connects directly to PostgreSQL database
- Runs on port 3008 by default
- TypeScript for type safety
- Minimal dependencies for lightweight deployment

The dashboard complements the proxy server by providing a user-friendly interface for monitoring and analyzing LLM API usage patterns.

README.md

@@ -148,6 +148,27 @@ Monitor your API usage through the PostgreSQL logs:
 - Identify usage patterns and optimize costs
 - Maintain compliance with audit requirements
 
+### Metrics Dashboard
+
+OpenProxy includes a lightweight Next.js dashboard for real-time metrics visualization:
+
+```bash
+cd dashboard
+npm install
+cp .env.example .env
+# Configure DATABASE_URL in .env
+npm run dev
+```
+
+The dashboard (available at `http://localhost:3008`) provides:
+- **Real-time Overview**: Total requests, tokens, costs, and response times
+- **Model Breakdown**: Usage statistics grouped by LLM model
+- **Hourly Trends**: Visual charts showing request patterns over time
+- **Recent Requests**: Detailed table of recent API calls
+- **Auto-refresh**: Automatic updates every 30 seconds
+
+See [dashboard/README.md](./dashboard/README.md) for detailed setup instructions.
+
 ## 🤝 Contributing
 
 Feel free to submit issues and enhancement requests!