Add user online presence tracking and notification

Introduced interfaces and services for tracking user online status, including IOnlineUserStore, IUserNotificationScopeService, and IUserPresenceReader. Added PresenceHub for real-time presence updates via SignalR. Updated OnlinePingingController to use new services and return online status and last seen. Extended UserDto with IsOnline property. Updated dependency injection and privacy settings enum. Removed obsolete IUserPresenceService.

Govor.Application/Interfaces/UserOnlineStatus/IOnlineUserStore.cs

@@ -0,0 +1,9 @@
+namespace Govor.Application.Interfaces.UserOnlineStatus;
+
+public interface IOnlineUserStore
+{
+    void SetOnlineUser(Guid userId);
+    void SetOfflineUser(Guid userId);
+    bool IsOnline(Guid userId);
+    IReadOnlyCollection<Guid> GetAllOnlineUsers();
+}

Govor.Application/Interfaces/UserOnlineStatus/IUserNotificationScopeService.cs

@@ -0,0 +1,6 @@
+namespace Govor.Application.Interfaces.UserOnlineStatus;
+
+public interface IUserNotificationScopeService
+{
+    Task<List<Guid>> GetNotifiedUsers(Guid userId);
+}

Govor.Application/Interfaces/UserOnlineStatus/IUserPresenceReader.cs

@@ -0,0 +1,6 @@
+namespace Govor.Application.Interfaces.UserOnlineStatus;
+
+public interface IUserPresenceReader
+{
+    Task<DateTime?> GetLastSeenAsync(Guid userId);
+}