Add Telegram notification for news publication to users

- Implemented NotifyUsersNewsPublishedAsync method in TelegramNotifier to send notifications to activated Telegram users when a news post is published.
- Updated CreatePostCommandHandler to invoke the new Telegram notification method after creating a news post.
- Enhanced IIdentityService to retrieve activated linked Telegram user IDs for notifications.
- Updated ITelegramNotifier interface to include the new notification method documentation.

backend/src/PnvPanel.Infrastructure/Identity/IdentityService.cs

@@ -252,6 +252,14 @@ internal sealed class IdentityService(UserManager<AppUser> userManager, SignInMa
             : new TelegramLinkInfo(false, null, null);
     }
 
+    public async Task<IReadOnlyCollection<long>> GetActivatedLinkedTelegramUserIdsAsync(CancellationToken cancellationToken)
+    {
+        return await userManager.Users.AsNoTracking()
+            .Where(u => u.TelegramUserId != null && u.IsActivated && !u.IsBlocked)
+            .Select(u => u.TelegramUserId!.Value)
+            .ToListAsync(cancellationToken);
+    }
+
     private async Task<string> GetPrimaryRoleNameAsync(AppUser user)
     {
         var roles = await userManager.GetRolesAsync(user);