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/Objects/NoteParser.cs

@@ -163,7 +163,7 @@ namespace PrivaPub.Federation.Objects
 		static DateTime? Time(string value) =>
 			DateTimeOffset.TryParse(value, CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal, out var parsed)
 				? parsed.UtcDateTime
-				: default;
+				: (DateTime?)null;
 
 		static int? Int(JsonNode node) =>
 			node is JsonValue value && value.TryGetValue<int>(out var number) && number is > 0 and < 100_000 ? number : (int?)null;