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/Federation/Rendering/ActivityPubRenderer.cs

@@ -48,7 +48,7 @@ namespace PrivaPub.Federation.Rendering
 			string.IsNullOrEmpty(markdown) ? string.Empty : Markdown.ToHtml(markdown, Pipeline).Trim();
 
 		public static string Timestamp(DateTime value) =>
-			DateTime.SpecifyKind(value, DateTimeKind.Utc).ToString("yyyy-MM-ddTHH:mm:ssZ", CultureInfo.InvariantCulture);
+			DateTime.SpecifyKind(value, DateTimeKind.Utc).ToString("yyyy-MM-ddTHH:mm:ss.fffZ", CultureInfo.InvariantCulture);
 
 		public static string Day(DateTime value) =>
 			DateTime.SpecifyKind(value, DateTimeKind.Utc).Date.ToString("yyyy-MM-ddT00:00:00Z", CultureInfo.InvariantCulture);