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/Domain/RelationshipTests.cs

@@ -62,7 +62,31 @@ namespace PrivaPub.Tests.Domain
 			});
 
 			Assert.False(await DB.Default.Find<Follower>().Match(f => f.LocalActorId == alice.Id).ExecuteAnyAsync(token));
-			Assert.Equal("Reject", Assert.Single(await _harness.Outgoing(mallory.Id + "/inbox"))["type"]!.GetValue<string>());
+			var sent = (await _harness.Outgoing(mallory.Id + "/inbox")).Select(a => a["type"]!.GetValue<string>()).ToList();
+			Assert.Equal(new[] { "Block", "Reject" }, sent.Order());
+		}
+
+		[Fact]
+		public async Task A_block_is_told_to_the_blocked_server_and_so_is_the_unblock()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (_, alice) = await _harness.Persona("alice");
+			var mallory = new RemoteActor(_harness.Peer, "mallory");
+			await _harness.Deliver(mallory, "/human-centipede", new JsonObject
+			{
+				["id"] = $"{Origin(mallory)}/follows/{Guid.NewGuid():N}", ["type"] = "Follow", ["actor"] = mallory.Id, ["object"] = alice.Uri
+			});
+
+			await _harness.Relationships.Block(alice, mallory.Id, default, token);
+			await _harness.Relationships.Unblock(alice, mallory.Id, token);
+
+			var sent = await _harness.Outgoing(mallory.Id + "/inbox");
+			var block = Assert.Single(sent, a => a["type"]!.GetValue<string>() == "Block");
+			Assert.Equal(mallory.Id, block["object"]!.GetValue<string>());
+			Assert.Equal(alice.Uri, block["actor"]!.GetValue<string>());
+			var undo = Assert.Single(sent, a => a["type"]!.GetValue<string>() == "Undo");
+			Assert.Equal(block["id"]!.GetValue<string>(), undo["object"]!["id"]!.GetValue<string>());
+			Assert.False(await DB.Default.Find<Block>().Match(b => b.AvatarId == alice.Id).ExecuteAnyAsync(token));
 		}
 
 		[Fact]