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/package.json

@@ -0,0 +1,26 @@
+{
+  "name": "openproxy-dashboard",
+  "version": "1.0.0",
+  "description": "Lightweight Next.js dashboard for OpenProxy metrics",
+  "private": true,
+  "scripts": {
+    "dev": "next dev -p 3008",
+    "build": "next build",
+    "start": "next start -p 3008",
+    "lint": "next lint"
+  },
+  "dependencies": {
+    "next": "^14.2.0",
+    "react": "^18.3.0",
+    "react-dom": "^18.3.0",
+    "pg": "^8.16.3",
+    "recharts": "^2.12.0"
+  },
+  "devDependencies": {
+    "@types/node": "^20.0.0",
+    "@types/react": "^18.3.0",
+    "@types/react-dom": "^18.3.0",
+    "@types/pg": "^8.11.0",
+    "typescript": "^5.9.0"
+  }
+}