Scheduled posts: written now, published at their time

POST /api/v1/statuses with scheduled_at no longer posts at once: the post is
kept as asked (at least five minutes ahead; 300 waiting, 25 a day, as
Mastodon allows), its media kept from the janitor, and a PublishScheduled job
publishes it at its time as the persona. scheduled_statuses lists, moves and
drops them; a moved post's old job finds it not due. Idempotency-Key holds
for scheduling too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Models/Media/MediaAttachment.cs

@@ -18,6 +18,7 @@ namespace PrivaPub.Models.Media
 		public float[] Focus { get; set; }
 		public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 		public DateTime? AttachedAt { get; set; }
+		public string ScheduledStatusId { get; set; }//kept for a scheduled post until it is published or dropped
 	}
 
 	public class MediaSecret : Entity