P7: thread backfill reads conversation containers and asks with If-None-Match

The FetchReplies job reads a thread's FEP-171b contextHistory before its FEP-7888 context: the posts each Add (or
Forte's plain Create) brought in, read from their own servers. A thread collection read whole keeps its ETag when the
document itself changes with every post (it counts them, or holds them all with no further page); the next read sends
it as If-None-Match and a 304 ends the job. The ETag is kept as sent, since NodeBB's has no quotes and the typed header
drops it. A context naming a post we hold (Forte's first post) is not fetched.

Checked in the pasture: Mastodon 4.7.3 answers the second read 304; NodeBB 4.16's unquoted ETag is kept (23/23 in its
scenario); a Forte thread completes through its replies.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

FEDERATION.md

@@ -394,9 +394,11 @@ Posts with a location (shown to nearby users of this server) never leave the ser
 - **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.
+  its replies are read from their servers, at most once an hour: the thread's conversation container (FEP-171b's
+  `contextHistory`: the posts its owner's `Add`s brought in) or its `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. A thread collection read whole, which counts its posts or
+  holds them all, is asked again with `If-None-Match` and its last ETag; a 304 ends the read.
 - **Reading our documents (SecureMode).** privapub.thepra.dev answers ActivityPub GETs only when they are signed, like
   Mastodon's authorized fetch; the server's own actors (the instance actor `/peasants/privapub` and the reporter
   `/peasants/privapub_reports`) are the exception, since their keys are needed first.