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/comments.js

@@ -61,10 +61,10 @@ router.post(
         content,
       });
 
-      // Emit Socket.IO event for new comment
-      const io = req.app.get("io");
-      if (io) {
-        io.to(`post_${postId}`).emit("newComment", {
+      // Emit SSE event for new comment
+      const sse = req.app.get("sse");
+      if (sse) {
+        sse.broadcastToTopic(`post_${postId}`, "newComment", {
           postId,
           comment,
         });
@@ -111,10 +111,10 @@ router.delete(
     // Delete comment
     await Comment.deleteComment(commentId);
 
-    // Emit Socket.IO event for deleted comment
-    const io = req.app.get("io");
-    if (io) {
-      io.to(`post_${postId}`).emit("commentDeleted", {
+    // Emit SSE event for deleted comment
+    const sse = req.app.get("sse");
+    if (sse) {
+      sse.broadcastToTopic(`post_${postId}`, "commentDeleted", {
         postId,
         commentId,
       });