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/Federation/Inbox/Handlers/CreateHandler.cs

@@ -5,6 +5,7 @@ using PrivaPub.Federation.Actors;
 using PrivaPub.Federation.Moderation;
 using PrivaPub.Federation.Objects;
 using PrivaPub.Federation.Outbox;
+using PrivaPub.Infrastructure.Ids;
 using PrivaPub.Federation.Rendering;
 using PrivaPub.Models.Federation;
 using PrivaPub.Models.Group;
@@ -113,6 +114,7 @@ namespace PrivaPub.Federation.Inbox.Handlers
 
 			var post = new PostEntity
 			{
+				ID = PrivacyIds.At(note.Published),
 				ObjectURI = note.Id,
 				ActivityURI = Id(activity),
 				ActorURI = author.ActorURI,