chore: add complete push notifications monitoring dashboard (#479)

docs/monitoring.md

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+# Push Notifications Monitoring Dashboard
+
+## Quick Start
+
+1. Run `npm install` then `npm run dev`
+2. Open `http://localhost:3000/system-health`
+3. Click "Enable Notifications" and allow permission
+4. Send a test message
+
+## Features
+
+- **System Health**: Real memory, CPU, uptime from Node.js
+- **Notification Metrics**: Sent, Delivered, Clicked, Failed counts
+- **Delivery Success Rate**: Visual progress bar
+- **Event Logs**: Complete history with filters and export
+- **Dark Mode**: Toggle light/dark themes
+- **Auto-Refresh**: Updates every 5 seconds
+
+## API Endpoints
+
+- `GET /api/notifications/metrics` - System metrics
+- `GET/POST/DELETE /api/notifications/track` - Event tracking
+- `POST /api/notifications/send-notification` - Send test notifications

lib/subscriptions.js

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+// Shared subscriptions storage for all API routes
+const subscriptions = new Map();
+
+export { subscriptions };