feat: Migrate from Socket.IO to Server-Sent Events (SSE)

- Replace Socket.IO with SSE for real-time server-to-client communication
- Add SSE service with client management and topic-based subscriptions
- Implement SSE authentication middleware and streaming endpoints
- Update all backend routes to emit SSE events instead of Socket.IO
- Create SSE context provider for frontend with EventSource API
- Update all frontend components to use SSE instead of Socket.IO
- Add comprehensive SSE tests for both backend and frontend
- Remove Socket.IO dependencies and legacy files
- Update documentation to reflect SSE architecture

Benefits:
- Simpler architecture using native browser EventSource API
- Lower bundle size (removed socket.io-client dependency)
- Better compatibility with reverse proxies and load balancers
- Reduced resource usage for Raspberry Pi deployment
- Standard HTTP-based real-time communication

🤖 Generated with [AI Assistant]

Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/test-sse-demo.js

@@ -0,0 +1,76 @@
+/**
+ * SSE Demo Script
+ * Demonstrates the SSE service functionality
+ */
+
+const sseService = require('./services/sseService');
+
+console.log('=== SSE Service Demo ===\n');
+
+// Mock response objects
+const createMockRes = (userId) => ({
+  write: (data) => console.log(`[${userId}] Received:`, data.trim()),
+});
+
+// 1. Add clients
+console.log('1. Adding clients...');
+const res1 = createMockRes('user1');
+const res2 = createMockRes('user2');
+const res3 = createMockRes('user3');
+
+sseService.addClient('user1', res1);
+sseService.addClient('user2', res2);
+sseService.addClient('user3', res3);
+
+let stats = sseService.getStats();
+console.log('Stats after adding clients:', stats);
+console.log('');
+
+// 2. Subscribe to topics
+console.log('2. Subscribing to topics...');
+sseService.subscribe('user1', ['events', 'posts']);
+sseService.subscribe('user2', ['events']);
+sseService.subscribe('user3', ['posts']);
+
+stats = sseService.getStats();
+console.log('Stats after subscriptions:', stats);
+console.log('');
+
+// 3. Broadcast to all
+console.log('3. Broadcasting to all clients...');
+sseService.broadcast('announcement', { message: 'Hello everyone!' });
+console.log('');
+
+// 4. Broadcast to topic
+console.log('4. Broadcasting to "events" topic (user1, user2)...');
+sseService.broadcastToTopic('events', 'eventUpdate', { eventId: 123, status: 'started' });
+console.log('');
+
+console.log('5. Broadcasting to "posts" topic (user1, user3)...');
+sseService.broadcastToTopic('posts', 'newPost', { postId: 456, author: 'Alice' });
+console.log('');
+
+// 6. Send to specific user
+console.log('6. Sending to specific user (user2)...');
+sseService.sendToUser('user2', 'notification', { text: 'You have a new message' });
+console.log('');
+
+// 7. Unsubscribe
+console.log('7. Unsubscribing user1 from "events"...');
+sseService.unsubscribe('user1', ['events']);
+stats = sseService.getStats();
+console.log('Stats after unsubscribe:', stats);
+console.log('');
+
+// 8. Remove client
+console.log('8. Removing user3...');
+sseService.removeClient('user3');
+stats = sseService.getStats();
+console.log('Stats after removing user3:', stats);
+console.log('');
+
+// Clean up
+sseService.removeClient('user1');
+sseService.removeClient('user2');
+
+console.log('=== Demo Complete ===');