Persona and group ids say which day, nothing more

An ObjectId carries its creation second and a per-process counter, so two
avatars made one after the other by the same login got ids a few counts
apart: a link between personas that every Mastodon client would have
seen. Avatar and Group now generate ids from the UTC day plus eight random
bytes (still valid ObjectIds), and a remote post's id is made from its
published time with a random tail, so posts page in the order they were
written rather than the order they arrived.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Models/Group/Group.cs

@@ -1,5 +1,7 @@
 using MongoDB.Entities;
 
+using PrivaPub.Infrastructure.Ids;
+
 namespace PrivaPub.Models.Group
 {
 	public class Group : Entity
@@ -29,6 +31,8 @@ namespace PrivaPub.Models.Group
 		public DateTime CreationDate { get; set; } = DateTime.UtcNow;
 		public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
 		public DateTime? DeletionAt { get; set; }
+
+		public override object GenerateNewID() => PrivacyIds.ForDay(DateTime.UtcNow);
 	}
 
 	public class GroupMember