feat: implement notification system with repository, service, and controller layers

Distribution/Features/Notifications/NotificationHub.cs

@@ -0,0 +1,22 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.SignalR;
+
+namespace Distribution.Features.Notifications;
+
+/// <summary>
+/// Server → client only. The server pushes a "notification" event to each recipient; clients never
+/// invoke hub methods. Users are addressed by their id (see <see cref="NotificationUserIdProvider"/>).
+/// </summary>
+[Authorize]
+public class NotificationHub : Hub
+{
+    public const string Path = "/hubs/notifications";
+    public const string ClientEvent = "notification";
+}
+
+/// <summary>Maps a SignalR connection to the "UserId" claim that <c>AuthMiddleware</c> puts on the principal.</summary>
+public class NotificationUserIdProvider : IUserIdProvider
+{
+    public string? GetUserId(HubConnectionContext connection) =>
+        connection.User?.FindFirst("UserId")?.Value;
+}