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/Infrastructure/PrivacyIdsTests.cs

@@ -12,15 +12,18 @@ namespace PrivaPub.Tests.Infrastructure
 		[Fact]
 		public void Persona_ids_carry_only_the_day_and_share_nothing_else()
 		{
-			var first = (string)new Avatar().GenerateNewID();
-			var second = (string)new Avatar().GenerateNewID();
+			var avatar = new Avatar();
+			var first = (string)avatar.GenerateNewID();
+			var second = (string)new Avatar { PublishedOn = avatar.PublishedOn }.GenerateNewID();
+			var group = new GroupEntity();
 
 			Assert.True(ObjectId.TryParse(first, out var a));
 			Assert.True(ObjectId.TryParse(second, out var b));
-			Assert.Equal(DateTime.UtcNow.Date, a.CreationTime);
+			Assert.Equal(avatar.PublishedOn, a.CreationTime);
+			Assert.InRange(a.CreationTime, DateTime.UtcNow.Date.AddDays(-13), DateTime.UtcNow.Date);
 			Assert.Equal(a.CreationTime, b.CreationTime);
 			Assert.NotEqual(first[8..14], second[8..14]);
-			Assert.Equal(DateTime.UtcNow.Date, ObjectId.Parse((string)new GroupEntity().GenerateNewID()).CreationTime);
+			Assert.Equal(group.PublishedOn, ObjectId.Parse((string)group.GenerateNewID()).CreationTime);
 		}
 
 		[Fact]
@@ -49,6 +52,17 @@ namespace PrivaPub.Tests.Infrastructure
 		public void A_backfilled_post_keeps_its_published_time() =>
 			Assert.Equal(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc), ObjectId.Parse(PrivacyIds.Arrived(new DateTime(2025, 1, 1, 0, 0, 0, DateTimeKind.Utc))).CreationTime);
 
+		[Fact]
+		public void A_persona_publishes_a_day_within_two_weeks_before_its_creation()
+		{
+			var created = new DateTime(2026, 10, 1, 15, 30, 0, DateTimeKind.Utc);
+			var days = Enumerable.Range(0, 200).Select(_ => PrivacyIds.PublishedDay(created)).ToList();
+
+			Assert.All(days, d => Assert.Equal(TimeSpan.Zero, d.TimeOfDay));
+			Assert.All(days, d => Assert.InRange(d, created.Date.AddDays(-13), created.Date));
+			Assert.True(days.Distinct().Count() > 1);
+		}
+
 		[Fact]
 		public void A_published_time_in_the_future_is_clamped() =>
 			Assert.True(ObjectId.Parse(PrivacyIds.At(DateTime.UtcNow.AddYears(50))).CreationTime < DateTime.UtcNow.AddDays(2));