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

docs/INTEROP.md

@@ -96,7 +96,9 @@ Priorities, used throughout:
   - `Move`.
   - `QuoteRequest`, its `Accept`/`Reject` (with `result`), and `Delete{QuoteAuthorization}`.
   - `FeatureRequest`/`FeatureAuthorization` (4.6).
-- **Threads:** `context` is a dereferenceable collection of the thread (FEP-7888, threads started on 4.5+).
+- **Threads:** `context` is a dereferenceable collection of the thread (FEP-7888, threads started on 4.5+). It
+  answers `If-None-Match` with a 304 (a weak ETag over the body, 4.7.3), but the body embeds only the first page, so its
+  ETag stands for the thread only while the thread fits on that page.
 - **Interaction counts:** `likes` and `shares` carry `totalItems`.
 - **Quotes:** `quote`, plus `quoteUri` and `_misskey_quote`, `quoteAuthorization`, and `interactionPolicy.canQuote`.
 - **Link previews:** from 4.7, a `{type: Link, href}` **attachment** names the link a card is made from (FEP-8967).
@@ -659,7 +661,9 @@ What it showed:
 - **NodeBB:**
   - A topic's first post is an `Article` with `name`, **`summary` = an excerpt** and `preview`; replies are Notes.
   - Categories are `Group`s **without `followers`**.
-  - `context` is a paged collection with an **ETag digest**; NodeBB refetches with `If-None-Match`.
+  - `context` is a collection with an **ETag digest** of its posts, sent **without quotes** (not an RFC 9110
+    entity-tag: .NET's typed header drops it, so PrivaPub keeps the raw value); it carries `totalItems` and its items
+    inline, and answers `If-None-Match` with a 304. NodeBB refetches with it too.
   - Since 4.15, an Announce of anything but a Create or a plain object is accepted only from Group actors.
   - It sends `Move`/`Remove` of a whole context (FEP-f15d) and `Add{post → context}` (FEP-11dd).
   - Chats are private Notes, threaded by `inReplyTo` into the room they answer.
@@ -855,6 +859,8 @@ without a port, before it gives out its OAuth client.
 - Its Mastodon API resolves an account elsewhere only when the search is not limited to a type
   (`/api/v2/search?resolve=true`, no `type=accounts`); reactions go through Pleroma's route
   (`PUT /api/v1/pleroma/statuses/:id/reactions/:emoji`).
+- **Threads:** a post's `context` is `/collections/conversations/<id>`, a collection whose first page lists the
+  thread's post ids (FEP-7888), with no ETag; no `contextHistory`.
 - **Pasture evidence (2026-10-05, `tools/pasture/scenarios/mitra.sh`):** 28 checks pass, with no change to PrivaPub:
   follows both ways, posts, replies both ways, likes and reposts both ways, an emoji reaction, a poll and alice's vote,
   edits and deletions both ways, direct messages both ways, the unfollow, statistics.
@@ -918,6 +924,11 @@ without a port, before it gives out its OAuth client.
 - **Everything in a thread is added to its conversation** (FEP-171b): its edits come only as `Add{Update}`, under the
   same activity id as the post's `Create`, and likes as `Add{Like}`. PrivaPub unwraps Create, Update and Delete, telling
   an Update from the Create whose id it reuses by what it carries.
+- **Its conversation container is not named by its posts:** `contextHistory` (and `context`) pointing at
+  `…/conversation/<uuid>` ride on the activities only; a Note's `context` is the thread's first post and it has no
+  `contextHistory`, though the FEP asks the first post for one. The container (public threads: unsigned GETs work)
+  lists the first post's plain `Create`, then an `Add{Create}` for every post. A post's `replies` lists every comment of
+  the thread, embedded, and is what PrivaPub reads to complete a Forte thread (checked live 2026-10-07).
 - **Its own collections name themselves wrongly:** a channel's followers, following and outbox give an `id` with the
   gateway path twice and a trailing `?`, which 404s, so their counts are not read.
 - **Running it:** no image is published; `images/forte` builds its tag with composer on Hubzilla's PHP image, behind an

docs/ROADMAP.md

@@ -677,8 +677,11 @@ it, raw where it doesn't.
 
 #### P7 Threads, communities and the social graph
 - **Thread backfill:**
-  - read in order: `contextHistory`, then `context` (paged, with ETag), then `replies`. **Done (2026-10-05), but for
-    `contextHistory` and ETags:** a persona opening a public remote thread queues `FetchReplies` for the post and its
+  - read in order: `contextHistory`, then `context` (paged, with ETag), then `replies`. **Done (2026-10-05; `contextHistory`
+    and ETags 2026-10-07: a container's `Add`ed Creates and Updates are read for their posts; a whole read of a collection
+    that counts its posts or holds them all keeps its ETag, sent back as `If-None-Match`, a 304 checked live on Mastodon
+    4.7.3 and NodeBB 4.16, whose ETag has no quotes; neither Forte nor Mitra puts `contextHistory` on a post, and Forte's
+    `replies` already holds its whole thread):** a persona opening a public remote thread queues `FetchReplies` for the post and its
     root, at most hourly each. The job reads the thread's FEP-7888 `context` collection (posts or, as FEP-f228 allows,
     the activities that made them), else the post's `replies` (PeerTube's `comments`) and theirs, two levels down. It
     reads at most 5 pages and 100 posts per job, signed by the instance actor, and stores what it lacks through