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.Tests/Federation/NoteParserTests.cs

@@ -54,6 +54,7 @@ namespace PrivaPub.Tests.Federation
 			Assert.Equal("a cat", media.Description);
 			Assert.Equal(new[] { 0.5f, -1f }, media.Focus);
 			Assert.Equal(new DateTime(2026, 9, 30, 10, 0, 0, DateTimeKind.Utc), note.Published);
+			Assert.Null(note.Updated);
 		}
 
 		[Fact]