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/Dockerfile

@@ -1,5 +1,5 @@
 # Multi-stage build for multi-architecture support (AMD64, ARM64)
-FROM --platform=$BUILDPLATFORM node:20-alpine AS builder
+FROM --platform=$BUILDPLATFORM node:20-alpine3.20 AS builder
 
 WORKDIR /app
 
@@ -13,7 +13,7 @@ RUN npm ci --production
 COPY . .
 
 # --- Production stage ---
-FROM --platform=$TARGETPLATFORM node:20-alpine
+FROM --platform=$TARGETPLATFORM node:20-alpine3.20
 
 # Install curl for health checks and other utilities
 RUN apk add --no-cache curl wget