feat: Add real-time notification center with WebSocket integration

- Add in-app notification system with database storage
- Implement bell icon component with unread count badge in dashboard header
- Add full Notifications page tab with pagination and filtering (All/Unread/Read)
- Implement real-time WebSocket push notifications to all connected clients
- Add notification API endpoints: GET, PUT /read, DELETE, POST /clear
- Fix critical bug in notificationService where 'status' variable was undefined
- Add Notifications route to App.js routing
- Enhanced UI with notification popup in header and dedicated full-page view
- Auto-reconnect WebSocket with 5s timeout on disconnect
- Support rich notification content with resource details and timestamps

server/metrics.js

@@ -0,0 +1,128 @@
+const db = require('./database');
+
+class MetricsCollector {
+  constructor() {
+    this.apiMetrics = new Map();
+    this.errorCounts = new Map();
+    this.startTime = Date.now();
+  }
+
+  // Track API request metrics
+  trackApiRequest(endpoint, method, statusCode, duration) {
+    const key = `${method}:${endpoint}`;
+    if (!this.apiMetrics.has(key)) {
+      this.apiMetrics.set(key, {
+        endpoint,
+        method,
+        totalRequests: 0,
+        successCount: 0,
+        errorCount: 0,
+        totalDuration: 0,
+        minDuration: Infinity,
+        maxDuration: 0,
+        statusCodes: {}
+      });
+    }
+
+    const metrics = this.apiMetrics.get(key);
+    metrics.totalRequests++;
+    metrics.totalDuration += duration;
+    metrics.minDuration = Math.min(metrics.minDuration, duration);
+    metrics.maxDuration = Math.max(metrics.maxDuration, duration);
+
+    if (statusCode >= 200 && statusCode < 400) {
+      metrics.successCount++;
+    } else {
+      metrics.errorCount++;
+    }
+
+    metrics.statusCodes[statusCode] = (metrics.statusCodes[statusCode] || 0) + 1;
+  }
+
+  // Track application errors
+  trackError(error, context = {}) {
+    const errorKey = error.message || 'Unknown Error';
+    const errorData = {
+      message: error.message,
+      stack: error.stack,
+      context,
+      timestamp: new Date().toISOString(),
+      count: (this.errorCounts.get(errorKey) || 0) + 1
+    };
+
+    this.errorCounts.set(errorKey, errorData.count);
+
+    // Log to database
+    try {
+      db.prepare(`
+        INSERT INTO error_logs (error_message, stack_trace, context, occurred_at)
+        VALUES (?, ?, ?, datetime('now'))
+      `).run(
+        error.message,
+        error.stack || '',
+        JSON.stringify(context)
+      );
+    } catch (err) {
+      // Silent failure - don't crash on logging errors
+    }
+
+    return errorData;
+  }
+
+  // Get current metrics snapshot
+  getMetrics() {
+    const apiStats = [];
+    this.apiMetrics.forEach((metrics, key) => {
+      const avgDuration = metrics.totalRequests > 0 
+        ? Math.round(metrics.totalDuration / metrics.totalRequests)
+        : 0;
+      
+      apiStats.push({
+        endpoint: metrics.endpoint,
+        method: metrics.method,
+        totalRequests: metrics.totalRequests,
+        successRate: metrics.totalRequests > 0
+          ? ((metrics.successCount / metrics.totalRequests) * 100).toFixed(2)
+          : 0,
+        avgResponseTime: avgDuration,
+        minResponseTime: metrics.minDuration === Infinity ? 0 : metrics.minDuration,
+        maxResponseTime: metrics.maxDuration,
+        statusCodes: metrics.statusCodes
+      });
+    });
+
+    return {
+      uptime: Math.floor((Date.now() - this.startTime) / 1000),
+      apiStats: apiStats.sort((a, b) => b.totalRequests - a.totalRequests),
+      errorCount: Array.from(this.errorCounts.values()).reduce((sum, count) => sum + count, 0),
+      uniqueErrors: this.errorCounts.size
+    };
+  }
+
+  // Get recent errors from database
+  getRecentErrors(limit = 50) {
+    try {
+      return db.prepare(`
+        SELECT 
+          id,
+          error_message,
+          stack_trace,
+          context,
+          REPLACE(occurred_at, ' ', 'T') || 'Z' as occurred_at
+        FROM error_logs
+        ORDER BY occurred_at DESC
+        LIMIT ?
+      `).all(limit);
+    } catch (err) {
+      return [];
+    }
+  }
+
+  // Clear in-memory metrics (for testing/reset)
+  reset() {
+    this.apiMetrics.clear();
+    this.errorCounts.clear();
+  }
+}
+
+module.exports = new MetricsCollector();