A remote thread brings the replies that never reached us

The context of a remote post showed only what PrivaPub happened to hold:
replies from servers nobody here follows were never seen, and only the
ancestors were ever fetched. Now a persona opening a public remote thread
queues FetchReplies for the post and its root, at most once an hour each.

The job reads the thread's own collection first (FEP-7888 `context`, which
Mastodon 4.5+ serves with every reply at any depth; posts or, as FEP-f228
allows, the activities that made them), and otherwise the post's `replies`
(PeerTube's `comments`) and the replies' own, two levels down. At most 5
pages and 100 posts a job, signed by the instance actor, never a persona;
each post is fetched from its own origin and stored through StoreContext,
so only public and unlisted ones are kept.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

FEDERATION.md

@@ -213,6 +213,10 @@ Posts with a location (shown to nearby users of this server) never leave the ser
   deletes, must be on its actor's origin. An embedded object from another origin is fetched from that origin.
 - **Fetching.** All fetches are signed by the instance actor. They go only to public addresses, follow at most three
   redirects and read at most 1 MB.
+- **Threads.** A reply's missing parents are fetched, up to 10 levels. When someone here opens a public remote thread,
+  its replies are read from their servers, at most once an hour: the thread's `context` collection (FEP-7888) if it has
+  one, else its `replies` (PeerTube's `comments`) and theirs, two levels down; 5 pages and 100 posts at most. Only
+  public and unlisted replies are kept, each fetched from its own origin.
 - **Reading our documents (SecureMode).** privapub.thepra.dev answers ActivityPub GETs only when they are signed, like
   Mastodon's authorized fetch; the instance actor `/peasants/privapub` is the exception, since its key is needed first.
   A browser asking for HTML is redirected to the public page instead.