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/Federation/NoteParserTests.cs

@@ -90,6 +90,28 @@ namespace PrivaPub.Tests.Federation
 			Assert.Equal("https://k.example/notes/8", note.QuoteUri);
 		}
 
+		[Theory]
+		[InlineData("Article", false, false)]
+		[InlineData("Event", false, false)]
+		[InlineData("Video", false, false)]
+		[InlineData("Page", false, false)]
+		[InlineData("Note", false, true)]
+		[InlineData("Question", false, true)]
+		[InlineData("Article", true, true)]
+		public void A_summary_warns_only_on_notes_or_when_marked_sensitive(string type, bool sensitive, bool warns)
+		{
+			var note = NoteParser.Parse(JsonNode.Parse($$"""
+				{
+					"id": "https://w.example/?p=12", "type": "{{type}}", "name": "Twelve", "summary": "Saturday, 10:00 at the library",
+					"sensitive": {{(sensitive ? "true" : "false")}}, "attributedTo": "https://w.example/author", "content": "<p>body</p>"
+				}
+				"""));
+
+			Assert.Equal(warns, note.Sensitive);
+			Assert.Equal(warns ? "Saturday, 10:00 at the library" : null, note.SpoilerText);
+			Assert.Equal(warns ? null : "Saturday, 10:00 at the library", note.Excerpt);
+		}
+
 		[Theory]
 		[InlineData("""{ "id": "https://x.example/1", "type": "Person" }""")]
 		[InlineData("""{ "id": "not a uri", "type": "Note" }""")]