T8: inbox gaps, jobs, migrations and pages; a deleted remote account's posts are hidden

Owner decision (2026-10-03, "A remote account deletes itself"): its posts are kept but
hidden everywhere.
- Post.AuthorGone (additive bool). DeleteHandler's actor-delete branch sets it on every
  post whose ActorURI is the actor (one update-many), besides dropping its follows and
  timeline rows as before. RemotePosts.Build sets it on a post stored later for an
  account already marked Deleted.
- One rule in VisibilityPolicy: IsShown (not deleted, author not gone), IsPublic and
  CanSee exclude AuthorGone, plus Shown(post) for loaded posts.
- Lookups by id answer 404 through CanSee (statuses/:id and every sub-route, context,
  bookmarks, favourites, polls, reactions, search); provenance, account statuses,
  home/public/tag timelines, notifications, conversations, reblogged_by, the clientapi
  home and post/DM lists, a community's outbox and our Announces filter on IsShown or
  IsPublic; the Mastodon mapper never renders a hidden post or a boost of one.

Tests (30 new):
- AuthorGoneTests: the rule, the handler (posts kept, boosts included, follows and rows
  gone), a post fetched after the delete, and 20 Mastodon/ActivityPub lookups over HTTP
  seen before and hidden after.
- InboxGapTests: actor Update refresh (name, sanitised summary, key rotation in place
  and to a new key id) even with an older `updated`; Undo{Follow} by activity id and by
  object; Reject of our QuoteRequest (and a stranger's ignored); group-wrapped
  Announce{Like} and Announce{Undo{Like}}; a locked persona's pending follow,
  FollowRequest notification, and Decide accepting and rejecting with the original Follow.
- JobHandlerTests: AncestorsJobHandler up to its depth limit; PollRefreshJob and
  PollCloseJob (local and remote polls); InstanceDescriber from a peer's NodeInfo and
  the weekly dedupe through ObjectRecords; LinkPreviews for public posts only;
  DeliveryJobHandler outcomes (2xx, 404/410, 429/503 with Retry-After in seconds and as
  a date, 5xx) and a signature and Digest the peer can verify; MediaJanitor.Sweep;
  OAuthPruner.Prune.
- MigrationTests: _003, _004, _006 and _007 on seeded rows.
- PublicPagesTests: /@user and /@user/{id} (visibility, junk ids, exact CSP,
  Referrer-Policy and nosniff), circle 404, community page, the instance actor,
  ActivityPub redirects, and markup escaped in posts, titles and bios.

Production changes besides the rule:
- LinkPreviews.Handle re-checks that a post is still shown and public (the rule
  Wanted applies) before fetching anything; before, only enqueueing checked it.
- The legacy /clientapi post and DM lists no longer return soft-deleted posts.
- MediaJanitor.Sweep and OAuthPruner.Prune are the loop bodies, now public and tested.
- InstanceDescriber.Address: a protected virtual identity seam so a test can point
  the https NodeInfo addresses at a plain-http peer; production behaviour unchanged.

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

PrivaPub/Api/Mastodon/Mappers/MastodonMapper.cs

@@ -157,7 +157,7 @@ namespace PrivaPub.Api.Mastodon.Mappers
 			var originalIds = posts.Where(p => p.ReblogOfPostId != default).Select(p => p.ReblogOfPostId).Distinct().ToList();
 			var originals = originalIds.Count == 0
 				? new Dictionary<string, PostEntity>()
-				: (await _dbEntities.Posts.Match(p => originalIds.Contains(p.ID) && !p.DeletedAt.HasValue).ExecuteAsync(token)).ToDictionary(p => p.ID);
+				: (await _dbEntities.Posts.Match(p => originalIds.Contains(p.ID)).Match(VisibilityPolicy.IsShown).ExecuteAsync(token)).ToDictionary(p => p.ID);
 			var all = posts.Concat(originals.Values).ToList();
 
 			var mentionUris = all.SelectMany(p => p.Mentions).Where(m => !m.IsLocal).Select(m => m.ActorURI).Distinct().ToList();
@@ -245,7 +245,7 @@ namespace PrivaPub.Api.Mastodon.Mappers
 				: await FollowedLocalAuthors(viewerId, localAuthors, token);
 			var quotedIds = nested ? new List<string>() : all.Where(p => p.QuoteState == QuoteState.Accepted && p.QuotedPostId != default).Select(p => p.QuotedPostId).Distinct().ToList();
 			var quotedPosts = new List<PostEntity>();
-			foreach (var quotedPost in quotedIds.Count == 0 ? new List<PostEntity>() : await _dbEntities.Posts.Match(p => quotedIds.Contains(p.ID) && !p.DeletedAt.HasValue).ExecuteAsync(token))
+			foreach (var quotedPost in quotedIds.Count == 0 ? new List<PostEntity>() : await _dbEntities.Posts.Match(p => quotedIds.Contains(p.ID)).Match(VisibilityPolicy.IsShown).ExecuteAsync(token))
 				if (quotedPost.Visibility != PostVisibility.LocalGeo && await VisibilityPolicy.CanSee(quotedPost, viewerId, token))
 					quotedPosts.Add(quotedPost);
 			var quotedStatuses = quotedPosts.Count == 0
@@ -270,7 +270,7 @@ namespace PrivaPub.Api.Mastodon.Mappers
 			var mapped = new List<Status>();
 			foreach (var post in posts)
 			{
-				if (hidden.Contains(post.ActorURI) || post.ReblogOfPostId != default && originals.TryGetValue(post.ReblogOfPostId, out var hiddenOriginal)
+				if (!VisibilityPolicy.Shown(post) || hidden.Contains(post.ActorURI) || post.ReblogOfPostId != default && originals.TryGetValue(post.ReblogOfPostId, out var hiddenOriginal)
 					&& hidden.Contains(hiddenOriginal.ActorURI))
 					continue;
 				var status = Map(post);