M6: provenance fixes, stored features, community edits that are real edits

- A fetched ObjectRecord no longer wears the signature, key, signed headers and @context
  of the activity that caused the fetch, and its ReceivedAt is the fetch's own time. Its
  activity fields now name the trigger. Provenance answers signature null and
  fetchedBy "instance-actor". Migration _008 clears the old fetched records.
- ObjectRecords store their ObjectFeatures extensions and context namespaces (migration
  _009 fills older records in batches), so statistics can count them. Provenance reads the
  stored lists.
- ForeignAvatar.Features records what an actor document uses (ActorFeatures): featured,
  shared inbox, FEP-521a, FEP-8b32, identity proofs, Misskey fields, indexable,
  undiscoverable, locked, key size, and more.
- RemoteEdits.Apply and RemoteDeletes.Remove are shared by the Update, Delete and Announce
  handlers. A community-wrapped Update is now an edit only when newer, refreshes polls
  and media otherwise, revises the ObjectRecord and re-resolves quotes. A
  community-wrapped Delete now tombstones the object, removes its ObjectRecord, reblogs
  and timeline rows, and lowers the reply count. Any deleted quoting post lowers the
  quoted post's quote count.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub/Federation/Inbox/Handlers/DeleteHandler.cs

@@ -73,11 +73,7 @@ namespace PrivaPub.Federation.Inbox.Handlers
 				return;
 			}
 
-			await DB.Default.Update<DeletedObject>()
-				.Match(d => d.ObjectURI == objectUri)
-				.Modify(d => d.DeletedAt, DateTime.UtcNow)
-				.Option(o => o.IsUpsert = true)
-				.ExecuteAsync(token);
+			await RemoteDeletes.Tombstone(objectUri, token);
 			var post = await _dbEntities.Posts.Match(p => p.ObjectURI == objectUri && p.ActorURI == actor.ActorURI).ExecuteFirstAsync(token);
 			if (post == default)
 			{
@@ -87,14 +83,9 @@ namespace PrivaPub.Federation.Inbox.Handlers
 			}
 			Arrival.Accept("removed");
 			Arrival.About(post.ObjectType, post.Visibility, post.CreationDate);
-			await DB.Default.DeleteAsync<PostEntity>(post.ID);
-			await DB.Default.DeleteAsync<ObjectRecord>(r => r.PostId == post.ID || r.ObjectURI == objectUri);
-			await DB.Default.DeleteAsync<TimelineEntry>(e => e.PostId == post.ID || e.ReblogOfPostId == post.ID);
+			await RemoteDeletes.Remove(post, objectUri, token);
 			if (!string.IsNullOrEmpty(post.GroupId) && await _localActors.FindById(LocalActorKind.Group, post.GroupId, token) is { IsCircle: false } community)
 				await _groups.Announce(community, activity.AsObject(), post.ObjectURI, isNewPost: false, token);
-			await DB.Default.DeleteAsync<PostEntity>(p => p.ReblogOfPostId == post.ID);
-			if (!string.IsNullOrEmpty(post.AnsweringToPostId))
-				await DB.Default.Update<PostEntity>().MatchID(post.AnsweringToPostId).Modify(b => b.Inc(p => p.RepliesCount, -1)).ExecuteAsync(token);
 		}
 	}
 }