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/Api/Mastodon/Controllers/ProvenanceController.cs

@@ -64,15 +64,16 @@ namespace PrivaPub.Api.Mastodon.Controllers
 					Id = record.ActivityId, Type = record.ActivityType, Actor = record.ActivityActorURI
 				},
 				Inbox = record?.Inbox,
-				Signature = record?.KeyId == default ? default : new ProvenanceSignature
+				FetchedBy = record?.Path == ObjectPath.Fetched ? "instance-actor" : default,
+				Signature = record?.KeyId == default || record.Path == ObjectPath.Fetched ? default : new ProvenanceSignature
 				{
 					Scheme = record.SignatureScheme, KeyId = record.KeyId, Algorithm = record.Algorithm, Headers = record.SignedHeaders
 				},
 				ReceivedAt = record == default ? default : MastodonJson.Time(record.ReceivedAt),
 				Published = record?.Published is { } published ? MastodonJson.Time(published) : default,
 				Updated = record?.Updated is { } updated ? MastodonJson.Time(updated) : default,
-				Extensions = ObjectFeatures.Detect(raw),
-				ContextNamespaces = ObjectFeatures.Namespaces(raw, record?.ActivityContext),
+				Extensions = record?.Extensions ?? ObjectFeatures.Detect(raw),
+				ContextNamespaces = record?.ContextNamespaces ?? ObjectFeatures.Namespaces(raw, record?.ActivityContext),
 				Raw = raw,
 				RawBytes = record?.RawBytes ?? 0,
 				RawHash = record?.RawHash,
@@ -118,6 +119,7 @@ namespace PrivaPub.Api.Mastodon.Controllers
 			public bool Refetched { get; set; }
 			public ProvenanceActivity Activity { get; set; }
 			public string Inbox { get; set; }
+			public string FetchedBy { get; set; }
 			public ProvenanceSignature Signature { get; set; }
 			public string ReceivedAt { get; set; }
 			public string Published { get; set; }