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/routes/posts.js

@@ -63,6 +63,14 @@ router.post(
     // Invalidate posts cache
     invalidateCacheByPattern('/api/posts');
 
+    // Emit SSE event for new post
+    const sse = req.app.get("sse");
+    if (sse) {
+      sse.broadcastToTopic("posts", "newPost", {
+        post,
+      });
+    }
+
     res.json({
       post,
       pointsAwarded: 5, // Standard post creation points
@@ -132,6 +140,16 @@ router.put(
 
     const updatedPost = await Post.addLike(req.params.id, req.user.id);
 
+    // Emit SSE event for post like
+    const sse = req.app.get("sse");
+    if (sse) {
+      sse.broadcastToTopic("posts", "postUpdate", {
+        type: "post_liked",
+        postId: req.params.id,
+        likes: updatedPost.likes,
+      });
+    }
+
     res.json(updatedPost.likes);
   }),
 );