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.

dashboard/.env.example

@@ -0,0 +1,5 @@
+# PostgreSQL connection string (same as proxy server)
+DATABASE_URL=postgresql://user:password@localhost:5432/database
+
+# Database table name (default: llm_proxy)
+DATABASE_TABLE=llm_proxy