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/circuitBreaker.js

@@ -0,0 +1,62 @@
+// Circuit breaker implementation for API resilience
+class CircuitBreaker {
+  constructor(failureThreshold = 5, resetTimeout = 60000, errorFilter = null) {
+    this.failureThreshold = failureThreshold;
+    this.resetTimeout = resetTimeout;
+    this.errorFilter = errorFilter;
+    this.failureCount = 0;
+    this.lastFailureTime = null;
+    this.state = 'closed'; // closed, open, half-open
+  }
+
+  async execute(fn, fallback = null) {
+    if (this.state === 'open') {
+      if (Date.now() - this.lastFailureTime > this.resetTimeout) {
+        this.state = 'half-open';
+      } else {
+        if (fallback) return fallback();
+        throw new Error('Circuit breaker is open');
+      }
+    }
+
+    try {
+      const result = await fn();
+      if (this.state === 'half-open') {
+        this.close();
+      }
+      return result;
+    } catch (error) {
+      if (this.errorFilter && !this.errorFilter(error)) {
+        throw error;
+      }
+
+      this.failureCount++;
+      this.lastFailureTime = Date.now();
+
+      if (this.failureCount >= this.failureThreshold) {
+        this.open();
+      }
+
+      throw error;
+    }
+  }
+
+  close() {
+    this.failureCount = 0;
+    this.state = 'closed';
+  }
+
+  open() {
+    this.state = 'open';
+  }
+
+  getState() {
+    return {
+      state: this.state,
+      failureCount: this.failureCount,
+      lastFailureTime: this.lastFailureTime
+    };
+  }
+}
+
+module.exports = CircuitBreaker;