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/streets.js

@@ -177,6 +177,16 @@ router.put(
         }
       );
 
+      // Emit SSE event for street adoption
+      const sse = req.app.get("sse");
+      if (sse) {
+        sse.broadcastToTopic("streets", "streetUpdate", {
+          type: "street_adopted",
+          streetId: street._id,
+          userId: req.user.id,
+        });
+      }
+
       res.json({
         street,
         pointsAwarded: 50,