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/Infrastructure/Data/Migrations/_008_fetched_records_lose_the_trigger_signature.cs

@@ -0,0 +1,19 @@
+using MongoDB.Entities;
+
+using PrivaPub.Models.Federation;
+
+namespace PrivaPub.Infrastructure.Data.Migrations
+{
+	public class _008_fetched_records_lose_the_trigger_signature : IMigration
+	{
+		public async Task UpgradeAsync() =>
+			await DB.Default.Update<ObjectRecord>()
+				.Match(r => r.Path == ObjectPath.Fetched)
+				.Modify(r => r.SignatureScheme, null)
+				.Modify(r => r.KeyId, null)
+				.Modify(r => r.Algorithm, null)
+				.Modify(r => r.SignedHeaders, new List<string>())
+				.Modify(r => r.ActivityContext, null)
+				.ExecuteAsync();
+	}
+}

PrivaPub/Infrastructure/Data/Migrations/_009_object_records_keep_their_extensions.cs

@@ -0,0 +1,41 @@
+using MongoDB.Entities;
+
+using PrivaPub.Federation.Objects;
+using PrivaPub.Models.Federation;
+
+using System.Text.Json;
+using System.Text.Json.Nodes;
+
+namespace PrivaPub.Infrastructure.Data.Migrations
+{
+	public class _009_object_records_keep_their_extensions : IMigration
+	{
+		public async Task UpgradeAsync()
+		{
+			while (true)
+			{
+				var batch = await DB.Default.Find<ObjectRecord>()
+					.Match(r => r.Extensions == null)
+					.Limit(500)
+					.ExecuteAsync();
+				if (batch.Count == 0)
+					return;
+				foreach (var record in batch)
+				{
+					JsonObject raw = default;
+					try
+					{
+						raw = record.Raw == default ? default : JsonNode.Parse(record.Raw) as JsonObject;
+					}
+					catch (JsonException)
+					{
+					}
+					await DB.Default.Update<ObjectRecord>().MatchID(record.ID)
+						.Modify(r => r.Extensions, ObjectFeatures.Detect(raw))
+						.Modify(r => r.ContextNamespaces, ObjectFeatures.Namespaces(raw, record.ActivityContext))
+						.ExecuteAsync();
+				}
+			}
+		}
+	}
+}