net10, the refactor finished, and federation that works

The tree had not compiled since its first commit: Group, DmGroup and
IGroupUsersService were referenced and never written, an IDE rename had
turned the user-settings DTO into the ViewAvatarServer enum, and the settings
were saved as an entity they no longer were.

Built now:
- Group (an ActivityPub Group actor with its own keys, members, invitation
  code and optional password) and DmGroup (a conversation), with
  /clientapi/group/{list,insert,update,join,leave,approve}.
- Posts and DMs: /clientapi/post/{list,insert,delete}, /clientapi/dm/{list,insert};
  DM recipients are local usernames or user@host handles resolved by WebFinger.
- Invitation sign-up and login against the group's invitation code, checking
  the password before any account is created.
- Federation: WebFinger, NodeInfo 2.0, actors at /peasants/{name} (Person,
  Group, and an Application instance actor) with SPKI keys, draft-cavage
  RSA-SHA256 HTTP signatures both ways, an inbox handling Follow (+Accept),
  Undo, Create, Delete and Update, an outbox, notes at /posts/{id}, and a
  persisted, retried, signed delivery queue. A post to a group is announced
  by the group to its followers (FEP-1b12).
- The unused, broken typed ActivityPub models are replaced by a renderer;
  NSign's HMAC setup, which could not federate, is gone.

Upgrade: net10.0, MongoDB.Entities 25.1 (instance DB API, Standard GUIDs),
Swashbuckle 10 / OpenApi 2, Serilog.AspNetCore 10, MailKit 4.18,
PasswordGenerator 3. The JWT keys are 64 bytes (IdentityModel 8 refuses
shorter for HS512). Production runs its own mongod on 127.0.0.1:27022, as
Sintopia's apps do, and deploys to privapub.thepra.dev from the build runner.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Services/Federation/ActivityPubRenderer.cs

@@ -0,0 +1,178 @@
+using Markdig;
+
+using PrivaPub.Models.Federation;
+
+using System.Globalization;
+using System.Text.Json.Nodes;
+
+using PostEntity = PrivaPub.Models.Post.Post;
+using DmPostEntity = PrivaPub.Models.Post.DmPost;
+
+namespace PrivaPub.Services.Federation
+{
+	public static class ActivityPubRenderer
+	{
+		public const string ActivityStreams = "https://www.w3.org/ns/activitystreams";
+		public const string Public = "https://www.w3.org/ns/activitystreams#Public";
+
+		static readonly MarkdownPipeline Pipeline = new MarkdownPipelineBuilder().DisableHtml().Build();
+
+		public static string Html(string markdown) =>
+			string.IsNullOrEmpty(markdown) ? string.Empty : Markdown.ToHtml(markdown, Pipeline).Trim();
+
+		public static string Timestamp(DateTime value) =>
+			DateTime.SpecifyKind(value, DateTimeKind.Utc).ToString("yyyy-MM-ddTHH:mm:ssZ", CultureInfo.InvariantCulture);
+
+		public static JsonObject Actor(LocalActor actor)
+		{
+			var document = new JsonObject
+			{
+				["@context"] = new JsonArray(ActivityStreams, "https://w3id.org/security/v1"),
+				["id"] = actor.Uri,
+				["type"] = actor.Kind switch
+				{
+					LocalActorKind.Group => "Group",
+					LocalActorKind.Application => "Application",
+					_ => "Person"
+				},
+				["preferredUsername"] = actor.UserName,
+				["name"] = actor.Name,
+				["summary"] = Html(actor.Summary),
+				["url"] = actor.Uri,
+				["inbox"] = actor.Inbox,
+				["outbox"] = actor.Outbox,
+				["followers"] = actor.Followers,
+				["following"] = actor.Following,
+				["published"] = Timestamp(actor.Published),
+				["manuallyApprovesFollowers"] = actor.ManuallyApprovesFollowers,
+				["discoverable"] = actor.Discoverable,
+				["endpoints"] = new JsonObject { ["sharedInbox"] = actor.SharedInbox },
+				["publicKey"] = new JsonObject
+				{
+					["id"] = actor.KeyId,
+					["owner"] = actor.Uri,
+					["publicKeyPem"] = Keys.ToSubjectPublicKeyInfoPem(actor.PublicKeyPem)
+				}
+			};
+			if (!string.IsNullOrEmpty(actor.PictureURL))
+				document["icon"] = new JsonObject { ["type"] = "Image", ["url"] = actor.PictureURL };
+			if (!string.IsNullOrEmpty(actor.ThumbnailURL))
+				document["image"] = new JsonObject { ["type"] = "Image", ["url"] = actor.ThumbnailURL };
+			return document;
+		}
+
+		public static JsonObject Note(PostEntity post, LocalActor author, LocalActor group, string inReplyTo)
+		{
+			var to = new JsonArray(Public);
+			var cc = new JsonArray(author.Followers);
+			if (group != default)
+				cc.Add(group.Uri);
+
+			var note = NoteBody(author.PostUri(post.ID), author, post.Title, post.Text, post.HasContentWarning,
+				post.CreationDate, to, cc, inReplyTo);
+			if (group != default)
+				note["audience"] = group.Uri;
+			return note;
+		}
+
+		public static JsonObject DirectNote(DmPostEntity post, LocalActor author, IReadOnlyList<(string Uri, string Handle)> recipients,
+			string context)
+		{
+			var note = NoteBody(author.PostUri(post.ID), author, post.Title, post.Text, post.HasContentWarning,
+				post.CreationDate, new JsonArray(recipients.Select(r => (JsonNode)r.Uri).ToArray()), new JsonArray(), default);
+			note["tag"] = new JsonArray(recipients.Select(r => (JsonNode)new JsonObject
+			{
+				["type"] = "Mention",
+				["href"] = r.Uri,
+				["name"] = "@" + r.Handle
+			}).ToArray());
+			note["content"] = string.Join(" ", recipients.Select(r => $"<span class=\"h-card\"><a href=\"{r.Uri}\" class=\"u-url mention\">@{r.Handle}</a></span>"))
+				+ " " + note["content"]!.GetValue<string>();
+			if (!string.IsNullOrEmpty(context))
+				note["context"] = context;
+			return note;
+		}
+
+		static JsonObject NoteBody(string id, LocalActor author, string title, string text, bool sensitive,
+			DateTime published, JsonArray to, JsonArray cc, string inReplyTo)
+		{
+			var note = new JsonObject
+			{
+				["id"] = id,
+				["type"] = "Note",
+				["attributedTo"] = author.Uri,
+				["content"] = Html(text),
+				["published"] = Timestamp(published),
+				["url"] = id,
+				["to"] = to,
+				["cc"] = cc,
+				["sensitive"] = sensitive
+			};
+			if (sensitive && !string.IsNullOrEmpty(title))
+				note["summary"] = title;
+			else if (!string.IsNullOrEmpty(title))
+				note["name"] = title;
+			if (!string.IsNullOrEmpty(inReplyTo))
+				note["inReplyTo"] = inReplyTo;
+			return note;
+		}
+
+		public static JsonObject Create(LocalActor actor, JsonObject note, string activityId) => new()
+		{
+			["@context"] = ActivityStreams,
+			["id"] = actor.ActivityUri(activityId),
+			["type"] = "Create",
+			["actor"] = actor.Uri,
+			["published"] = note["published"]?.DeepClone(),
+			["to"] = note["to"]?.DeepClone(),
+			["cc"] = note["cc"]?.DeepClone(),
+			["object"] = note
+		};
+
+		public static JsonObject Announce(LocalActor group, string objectUri, string activityId) => new()
+		{
+			["@context"] = ActivityStreams,
+			["id"] = group.ActivityUri(activityId),
+			["type"] = "Announce",
+			["actor"] = group.Uri,
+			["published"] = Timestamp(DateTime.UtcNow),
+			["to"] = new JsonArray(Public),
+			["cc"] = new JsonArray(group.Followers),
+			["object"] = objectUri
+		};
+
+		public static JsonObject Delete(LocalActor actor, string objectUri, string activityId, JsonArray to, JsonArray cc) => new()
+		{
+			["@context"] = ActivityStreams,
+			["id"] = actor.ActivityUri(activityId),
+			["type"] = "Delete",
+			["actor"] = actor.Uri,
+			["to"] = to,
+			["cc"] = cc,
+			["object"] = new JsonObject { ["id"] = objectUri, ["type"] = "Tombstone" }
+		};
+
+		public static JsonObject Accept(LocalActor actor, JsonNode follow, string activityId) => new()
+		{
+			["@context"] = ActivityStreams,
+			["id"] = actor.ActivityUri(activityId),
+			["type"] = "Accept",
+			["actor"] = actor.Uri,
+			["object"] = follow.DeepClone()
+		};
+
+		public static JsonObject OrderedCollection(string id, int totalItems, IEnumerable<JsonNode> items)
+		{
+			var collection = new JsonObject
+			{
+				["@context"] = ActivityStreams,
+				["id"] = id,
+				["type"] = "OrderedCollection",
+				["totalItems"] = totalItems
+			};
+			if (items != default)
+				collection["orderedItems"] = new JsonArray(items.ToArray());
+			return collection;
+		}
+	}
+}