T6: the Mastodon API over HTTP

88 integration tests drive the Mastodon client API through the whole server
(PrivaPubHost), with remote actors on an in-process Peer and deliveries read
from the job queue. Helpers live in Support/Host/MastodonHelpers.cs.

Coverage:
- Accounts: verify_credentials (no root id or login name); update_credentials
  with indexed and array fields_attributes (form and JSON), source[*],
  quote_policy, locked/bot, avatar and header uploads resized and stripped of
  EXIF and XMP; lookup (local, @domain, remote; a circle, the instance actor
  and a circle's id answer 404); search with and without resolve (only a
  signed-in persona resolves, the Peer is untouched otherwise), by post and
  actor address, hashtags, undiscoverable personas; account statuses with
  pinned, exclude_replies, exclude_reblogs, only_media, tagged and Link paging
  both ways; followers-only posts for followers (local and remote authors);
  community accounts; followers/following only to their owner; follow (open,
  locked, remote Follow delivery), unfollow and Undo; follow requests from
  local and remote followers answered with the original Follow;
  remove_from_followers; blocks with Reject and Block/Undo deliveries; mutes
  with duration and the notifications choice, never federated; domain blocks;
  relationships with junk ids; a banned login's tokens; reports forwarded as a
  Flag from the instance actor only; account stub routes.
- Statuses: each visibility's to/cc as delivered; CW as summary; replies to
  local and remote posts (mention, inReplyTo, the author's inbox); polls and
  votes (local, and remote votes only to the author without published); media
  attached only by its owner; quotes, the quotes list and revocation; edit
  history, source and the Update delivery; delete for redraft, the 410
  Tombstone and the Delete delivery; favourite/reblog counts with Like,
  Announce and their Undos; favourited_by, reblogged_by; bookmarks; pins;
  interaction_policy matching canQuote in the note and in the Update;
  strangers get 404 for followers-only and direct posts; a located post is
  unreachable by id for anyone else on every route; statuses?id[];
  Idempotency-Key; scopes; deleting a reblog.
- Timelines: home paging with max_id, since_id and min_id; public local and
  remote; tag (anonymous); list stub; favourites; conversations and read;
  markers; notifications with types[], exclude_types[], account_id, paging,
  get, dismiss, clear and unread_count.
- Instance: v1 and v2 (4.2.0 (compatible; PrivaPub)), peers, activity, rules,
  extended_description, apps and every stub route.
- Media: v1 and v2 uploads, owner-only GET and PUT, 422 for unsupported or
  unreadable files, video and audio made with ffmpeg lavfi sources and checked
  with ffprobe; every remote media address goes through the proxy; the proxy
  refuses unsigned URLs, streams ranges as 206 without caching, caches whole
  downloads and serves them with ranges, and streams anything over
  Media:MaxProxiedBytes (a SmallProxyHost) without caching.
- Provenance of local, delivered (signature) and fetched (instance actor,
  no signature, the trigger as activity) posts, visibility of provenance,
  instance descriptions; reading any of them makes no outbound request.
  Pleroma reactions with EmojiReact and Undo deliveries, and local reaction
  notifications.

Bugs fixed:
- remove_from_followers deleted the Follower row but never told a remote
  follower. It now sends Reject{Follow} with the stored Follow id, through
  RelationshipService.RemoveFollower, which Block now shares.
- VisibilityPolicy.CanSee refused followers-only posts to accepted followers,
  so a post in their home timeline answered 404 to GET, context, favourite and
  reply. Followers of the author (local or remote) may now see them.
- Account statuses of a remote account hid followers-only posts from
  personas that follow it.
- exclude_replies dropped the author's own threads; like Mastodon it now
  drops only replies to other accounts.
- A community account's statuses were always empty: they are now the posts
  addressed to the community.
- Pinning someone else's visible post answered 404; it answers 422 like
  Mastodon.
- GET /api/v1/notifications/:id answered 200 with null when the notification's
  post was gone; it answers 404.

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

PrivaPub/Domain/Privacy/VisibilityPolicy.cs

@@ -2,6 +2,7 @@ using MongoDB.Entities;
 
 using PrivaPub.Models.Group;
 using PrivaPub.Models.Post;
+using PrivaPub.Models.Social;
 
 using System.Linq.Expressions;
 
@@ -41,8 +42,20 @@ namespace PrivaPub.Domain.Privacy
 				PostVisibility.Circle => !string.IsNullOrEmpty(post.GroupId) && await DB.Default.Find<GroupEntity>()
 					.Match(g => g.ID == post.GroupId && g.Members.Any(m => !m.IsForeign && m.AvatarId == viewerAvatarId))
 					.ExecuteAnyAsync(token),
+				PostVisibility.FollowersOnly => await FollowsAuthor(viewerAvatarId, post, token),
 				_ => false
 			};
 		}
+
+		static async Task<bool> FollowsAuthor(string viewerAvatarId, Post post, CancellationToken token)
+		{
+			if (!string.IsNullOrEmpty(post.ActorURI))
+				return await DB.Default.Find<Following>()
+					.Match(f => f.AvatarId == viewerAvatarId && f.TargetActorURI == post.ActorURI && f.State == FollowState.Accepted)
+					.ExecuteAnyAsync(token);
+			return !post.IsFederatedCopy && !string.IsNullOrEmpty(post.GroupUserId) && await DB.Default.Find<Following>()
+				.Match(f => f.AvatarId == viewerAvatarId && f.TargetIsLocal && f.TargetAccountId == post.GroupUserId && f.State == FollowState.Accepted)
+				.ExecuteAnyAsync(token);
+		}
 	}
 }