Add SignalR integration for real-time weather updates

Implemented SignalR to enable real-time communication between the server and connected clients. Added a new hub (`WeatherUpdateHub`), a background service (`SignalRSendService`), and modified both the API backend and React frontend for seamless message broadcasting and handling.

Api/SignalR/WeatherUpdateHub.cs

@@ -0,0 +1,14 @@
+using Microsoft.AspNetCore.SignalR;
+
+namespace Api.SignalR
+{
+    public class WeatherUpdateHub: Hub
+    {
+        // This method can be called by connected clients.
+        public async Task SendMessage(string user, string message)
+        {
+            // Broadcast the message to all connected clients.
+            await Clients.All.SendAsync("ReceiveMessage", user, message);
+        }
+    }
+}