One sign-in for decePubClient: the root JWT exchanged for a persona's token

The first-party client signs in on /clientapi and exchanges that JWT on
/oauth/token (RFC 8693, subject_token_type jwt, avatar_id) for one
persona's Mastodon token. Only the seeded public application `decepub`
holds the grant; RootJwtSubjectToken validates the JWT through RootJwt,
which JwtBearer now shares (signature, lifetime, ban, deletion, session
stamp). The issued token names the avatar, never the root.

Owner decision recorded in ROADMAP; it supersedes "moving decePubClient
onto the Mastodon API is out of scope".

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

docs/ROADMAP.md

@@ -1,4 +1,4 @@
-# PrivaPub roadmap
+# PrivaPub roadmap
 
 Written 2026-10-01 from the original 2023 code, the decePubClient UI, a federation gap audit of commit `075c222`, research on .NET ActivityPub libraries, and the owner's decisions. Each phase ends in a tagged deploy plus verification; tick phases off here as they land.
 
@@ -151,7 +151,7 @@ and circles (see Owner decisions).
 
 | Question | Decision |
 |---|---|
-| Client interface | **Mastodon client API**, so Tusky, Elk, Phanpy, Ivory and the official apps work. Each avatar is its own Mastodon account: at OAuth authorize, the logged-in RootUser picks the avatar the token is for. PrivaPub-only features (avatars, groups, contacts, range posts) stay on `/clientapi`. Moving decePubClient onto the Mastodon API is out of scope. |
+| Client interface | **Mastodon client API**, so Tusky, Elk, Phanpy, Ivory and the official apps work. Each avatar is its own Mastodon account: at OAuth authorize, the logged-in RootUser picks the avatar the token is for. PrivaPub-only features (avatars, groups, contacts, range posts) stay on `/clientapi`. ~~Moving decePubClient onto the Mastodon API is out of scope.~~ *Superseded 2026-10-04: decePubClient uses both APIs with one sign-in, see "Owner decisions on decePubClient".* |
 | Personas | **Unlinkable to other users and servers.** The admin can still see the link in the database. No root IDs next to IPs in logs, NodeInfo counts nothing that links personas, blocks, mutes and notifications are per avatar, and invitation signup no longer names the avatar after the root username. |
 | Location-ranged posts | **Local only, never federated.** Shown to local users within the radius, with coordinates rounded on storage. |
 | Groups | **Per group, two kinds.** A *community* federates per FEP-1b12, Lemmy-compatible: it Announces the activity, uses `audience`, and accepts posts from non-followers. A *circle* is invitation-only: posts are addressed to the members collection, and objects are served only to signed requests from members. |
@@ -197,6 +197,12 @@ and circles (see Owner decisions).
 | What circles and located posts reveal | **Unchanged**: circles still answer WebFinger, and circle and located posts still count in a persona's post count, "a good balance for the fediverse to work". |
 | Public `/stargazing` statistics | **Later**, as decided on 2026-10-03; the crawler and the admin API keep collecting meanwhile. |
 
+### Owner decisions on decePubClient (2026-10-04)
+
+| Question | Decision |
+|---|---|
+| Sign-in | **One login.** decePubClient signs in on `/clientapi` and exchanges that JWT for one persona's Mastodon token (RFC 8693 token exchange on `/oauth/token`, `PersonaExchange`), one token per persona it uses. Only the seeded first-party application may exchange; the token names the persona, never the root, like any other. |
+
 ## Libraries (researched; no maintained .NET ActivityPub library exists, so Letterbook and Iceshrimp.NET both wrote their own)
 
 | Area | Choice |
@@ -621,7 +627,6 @@ report at least 10 users; smaller ones fold into one "small servers" aggregate.
   - S3 storage.
   - JSON-LD and LD-signature processing. FEP-8b32 proofs need neither (JCS), so they are in P8.
 - **Deferred:** video transcoding (remux only for now).
-- **Out of scope:** moving decePubClient onto the Mastodon API.
 
 ## Verification (per phase)