Add support for priority

Set default title to topic unique, as displayed in subscriptions list, if not provided
Increase toast timeout to 5 seconds

ntfysh_client/Notifications/NtfyEvent.cs

@@ -20,6 +20,9 @@ namespace ntfysh_client.Notifications
         public string Message { get; set; } = null!;
 
         [JsonProperty("title")]
-        public string Title { get; set; } = null!;
+        public string? Title { get; set; }
+        
+        [JsonProperty("priority")]
+        public NotificationPriority? Priority { get; set; }
     }
 }