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/Federation/Actors/LocalActorService.cs

@@ -202,7 +202,7 @@ namespace PrivaPub.Federation.Actors
 			ThumbnailURL = avatar.ThumbnailURL,
 			PrivateKeyPem = avatar.PrivateKey,
 			PublicKeyPem = avatar.PublicKey,
-			Published = avatar.CreatedAt,
+			Published = avatar.PublishedOn,
 			Fields = avatar.Fields ?? new Dictionary<string, string>(),
 			ManuallyApprovesFollowers = avatar.Settings?.IsLocked == true,
 			Discoverable = avatar.Settings?.IsDiscoverable != false,
@@ -227,7 +227,7 @@ namespace PrivaPub.Federation.Actors
 			ManuallyApprovesFollowers = group.Kind == GroupKind.Circle || group.ManuallyApprovesMembers,
 			IsCircle = group.Kind == GroupKind.Circle,
 			PostingRestrictedToModerators = group.PostingPolicy == PostingPolicy.Moderators,
-			Published = group.CreationDate,
+			Published = group.PublishedOn,
 			BaseAddress = BaseAddress
 		};
 	}