P5, first batch: summaries stop hiding articles, personas get reachable names, blocks federate

- A remote `summary` is a content warning only on a Note or Question, or when `sensitive` is set. On Articles,
  Events, Videos, Pages and Audio it is an excerpt (WordPress teasers, Mobilizon dates and places, Mbin titles) and
  is now kept as `Post.Excerpt` instead of hiding the post. `Post.ObjectType` records the remote type, and the
  Mastodon API shows a remote non-Note object's title above its body again.
- Persona usernames must match `^[a-z0-9_]+$`, as groups already did; a name outside it was unreachable from
  Mastodon and Misskey.
- `postingRestrictedToMods` is defined in our JSON-LD context (Iceshrimp.NET drops undefined terms).
- `Vary: Accept` on actor and object URLs.
- Owner decision: blocks federate. A block sends `Block`, an unblock `Undo{Block}`; checked live against GoToSocial.
- Owner decision: a persona's and a group's `published`, and the day in new ids, is a random day up to two weeks
  before creation, so personas made the same day no longer share a date. Migration _007 gives existing ones theirs.

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

PrivaPub.Tests/Api/StatusContentTests.cs

@@ -0,0 +1,29 @@
+using PrivaPub.Api.Mastodon.Mappers;
+using PrivaPub.Models.Post;
+
+namespace PrivaPub.Tests.Api
+{
+	public class StatusContentTests
+	{
+		[Fact]
+		public void A_remote_article_shows_its_title_above_its_body() =>
+			Assert.Equal("<p><strong>Rain &amp; sun</strong></p><p>body</p>", MastodonMapper.Content(new Post
+			{
+				IsFederatedCopy = true, ObjectType = "Article", Title = "Rain & sun", ContentHtml = "<p>body</p>"
+			}));
+
+		[Fact]
+		public void A_remote_note_keeps_its_content_alone() =>
+			Assert.Equal("<p>body</p>", MastodonMapper.Content(new Post
+			{
+				IsFederatedCopy = true, ObjectType = "Note", Title = "a poll choice", ContentHtml = "<p>body</p>"
+			}));
+
+		[Fact]
+		public void A_remote_event_without_a_body_shows_its_excerpt() =>
+			Assert.Equal("<p><strong>Meetup</strong></p><p>Saturday &lt;10:00&gt;</p>", MastodonMapper.Content(new Post
+			{
+				IsFederatedCopy = true, ObjectType = "Event", Title = "Meetup", Excerpt = "Saturday <10:00>", ContentHtml = string.Empty
+			}));
+	}
+}