Everything on, phase 4b: ids that resolve, a hashtag page, grouped notifications, remote accounts' real counts

- A community's announces resolve, as FEDERATION.md and ROADMAP always said Announce ids do. GroupDistributor keeps
  each one it sends (GroupAnnouncement, unique by id). /grunts serves it while the post it carries is shown and 410
  after, and also serves the announce-{postId} ids the group outbox lists, which now keep a stable `published`
  instead of the time of the fetch.
- A persona's boost points at the boosted post: its `url` in the Mastodon API is the boosted post's page, and a browser
  following the boost's id is redirected there instead of getting JSON.
- /tags/{tag}, where every Hashtag link we send points, is now a public page of this server's public posts with that
  tag, with the same strict CSP and noindex as the profile pages.
- Grouped notifications (/api/v2/notifications, its unread count, a group, its accounts and dismiss). We advertise
  api_versions.mastodon = 7 so clients show quotes, and clients that trust it call these; they answered 404. Likes and
  boosts of one post group together, as do follows within an hour. The version string stays 4.2.0 until streaming
  and Web Push exist.
- A remote account's follower, following and post counts are what its server publishes. AccountCountsJob reads its
  collections' totalItems from its own origin, signed by the instance actor, at most daily and only after the account
  was fetched, never when someone looks. They used to be 0.
- The other ids that do not resolve are documented as such: Update, Delete, EmojiReact, QuoteRequest and its answers,
  Flag, Ignore and poll votes.

676 tests pass.

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

PrivaPub/Federation/Outbox/GroupDistributor.cs

@@ -1,5 +1,8 @@
+using MongoDB.Entities;
+
 using PrivaPub.Federation.Actors;
 using PrivaPub.Federation.Rendering;
+using PrivaPub.Models.Federation;
 
 using System.Security.Cryptography;
 using System.Text;
@@ -14,6 +17,26 @@ namespace PrivaPub.Federation.Outbox
 
 	public class GroupDistributor : IGroupDistributor
 	{
+		// so the announce's id resolves (PeasantsController.Grunt); a repeat of the same announce keeps the first
+		static async Task Keep(LocalActor group, JsonObject announce, string objectUri, CancellationToken token)
+		{
+			var body = (JsonObject)announce.DeepClone();
+			body.Remove("@context");
+			try
+			{
+				await DB.Default.SaveAsync(new GroupAnnouncement
+				{
+					GroupId = group.Id,
+					ActivityURI = body["id"]!.GetValue<string>(),
+					ObjectURI = objectUri,
+					Body = body.ToJsonString()
+				}, token);
+			}
+			catch (MongoDB.Driver.MongoWriteException ex) when (ex.WriteError?.Category == MongoDB.Driver.ServerErrorCategory.DuplicateKey)
+			{
+			}
+		}
+
 		readonly IDeliveryService _delivery;
 
 		public GroupDistributor(IDeliveryService delivery)
@@ -43,8 +66,13 @@ namespace PrivaPub.Federation.Outbox
 			// Every follower, the author's own server included: Lemmy 1.0 keeps its user's post pending until the community
 			// announces it back, and clears that before it answers the echo 400 ("Object is not remote").
 			await _delivery.EnqueueToFollowers(group, announce, token);
+			await Keep(group, announce, objectUri, token);
 			if (isNewPost && !string.IsNullOrEmpty(objectUri))
-				await _delivery.EnqueueToFollowers(group, ActivityPubRenderer.Announce(group, objectUri, $"announce-object-{key}"), token);
+			{
+				var boost = ActivityPubRenderer.Announce(group, objectUri, $"announce-object-{key}");
+				await _delivery.EnqueueToFollowers(group, boost, token);
+				await Keep(group, boost, objectUri, token);
+			}
 		}
 	}
 }