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/package.json

@@ -32,7 +32,6 @@
     "multer": "^1.4.5-lts.1",
     "nano": "^10.1.4",
     "node-cache": "^5.1.2",
-    "socket.io": "^4.8.1",
     "stripe": "^17.7.0",
     "xss-clean": "^0.1.4"
   },
@@ -42,7 +41,6 @@
     "eslint": "^9.38.0",
     "jest": "^30.2.0",
     "jest-environment-node": "^30.2.0",
-    "socket.io-client": "^4.8.1",
     "supertest": "^7.1.4"
   }
 }