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/Web/Pages/Pages.cs

@@ -92,6 +92,49 @@ namespace PrivaPub.Web.Pages
 		}
 	}
 
+	// The hashtag links our posts carry (/tags/{tag}): this server's public posts with that tag, as a page like a profile's.
+	public class TagModel : PublicPageModel
+	{
+		const int PageSize = 20;
+
+		readonly ILocalActorService _localActors;
+		readonly DbEntities _dbEntities;
+
+		public TagModel(ILocalActorService localActors, DbEntities dbEntities)
+		{
+			_localActors = localActors;
+			_dbEntities = dbEntities;
+		}
+
+		public string Tag { get; private set; }
+		public IReadOnlyList<PostView> Posts { get; private set; } = Array.Empty<PostView>();
+
+		public async Task<IActionResult> OnGetAsync(string tag, CancellationToken token)
+		{
+			Tag = tag?.TrimStart('#').ToLowerInvariant();
+			if (string.IsNullOrEmpty(Tag) || Tag.Length > 100)
+				return NotFound();
+			Harden();
+			var posts = await _dbEntities.Posts
+				.Match(p => p.Tags.Contains(Tag) && !p.IsFederatedCopy && p.ReblogOfPostId == null && p.Visibility == PostVisibility.Public)
+				.Match(VisibilityPolicy.IsPublic)
+				.Sort(p => p.ID, Order.Descending)
+				.Limit(PageSize)
+				.ExecuteAsync(token);
+			var authors = new Dictionary<string, LocalActor>();
+			var views = new List<PostView>();
+			foreach (var post in posts)
+			{
+				if (!authors.TryGetValue(post.GroupUserId, out var author))
+					authors[post.GroupUserId] = author = await _localActors.FindById(LocalActorKind.Person, post.GroupUserId, token);
+				if (author is { IsFederated: true })
+					views.Add(PostView.From(post, author));
+			}
+			Posts = views;
+			return Page();
+		}
+	}
+
 	public class StatusModel : PublicPageModel
 	{
 		readonly ILocalActorService _localActors;

PrivaPub/Web/Pages/Tag.cshtml

@@ -0,0 +1,17 @@
+@page "/tags/{tag}"
+@model PrivaPub.Web.Pages.TagModel
+@{
+	ViewData["Title"] = $"#{Model.Tag}";
+}
+<header>
+	<h1>#@Model.Tag</h1>
+	<div class="meta">Public posts on this server with this tag.</div>
+</header>
+@foreach (var post in Model.Posts)
+{
+	<partial name="_Post" model="post" />
+}
+@if (Model.Posts.Count == 0)
+{
+	<p class="meta">Nothing public with this tag yet.</p>
+}