Remote posts take their timeline place on arrival, and an unedited one has no edit date

Found by the first live run against GoToSocial:

- A remote post's id came from its published second plus random bytes, so a reply arriving in the same second as
  the post it answers could sort under it, and a late arrival landed behind a client's since_id. A post published
  within the last hour now gets an id for its arrival; backfill keeps its published time.
- NoteParser.Time returned default(DateTime) for a missing "updated", so every remote post was stored as edited in
  year one. Migration _006 clears the stored ones.
- published now carries milliseconds, so peers that derive ids from it (GoToSocial, Mastodon) keep our posts in
  order within a second.

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

PrivaPub/Infrastructure/Ids/PrivacyIds.cs

@@ -1,12 +1,19 @@
 using System.Buffers.Binary;
 using System.Security.Cryptography;
 
+using MongoDB.Bson;
+
 namespace PrivaPub.Infrastructure.Ids
 {
 	public static class PrivacyIds
 	{
 		public static string ForDay(DateTime when) => At(DateTime.SpecifyKind(when, DateTimeKind.Utc).Date);
 
+		public static string Arrived(DateTime published) =>
+			DateTime.SpecifyKind(published, DateTimeKind.Utc) > DateTime.UtcNow - LiveWindow ? ObjectId.GenerateNewId().ToString() : At(published);
+
+		static readonly TimeSpan LiveWindow = TimeSpan.FromHours(1);
+
 		public static string At(DateTime when)
 		{
 			var seconds = Math.Clamp(new DateTimeOffset(DateTime.SpecifyKind(when, DateTimeKind.Utc)).ToUnixTimeSeconds(), 0,