feat(user-service): cross-site thread-unread badge RPC

Adds a client-facing RPC that returns a per-user thread-unread badge
aggregated across every federation site the user participates in, and
removes the superseded (callerless) ThreadUnreadSummary leaf.

New client RPC: chat.user.{account}.request.user.{siteID}.thread.unread
-> ThreadUnreadResponse{ unread, unreadDirectMessage, unreadMention,
lastMessageAt?, unavailableSites? }.

How it works (mirrors the existing subscription-unread pattern, one
unread() helper): user-service reads all of the user's thread-subs from
the home-site replica (ThreadSubscriptionRepo.ListByAccount), groups them
by the room's home siteId, and per site (uncapped fan-out, chunked by
maxBatchSize) calls the new room-service ThreadRoomInfoBatch RPC
(chat.server.request.room.{siteID}.thread.info.batch) for each thread
room's lastMsgAt + parent roomType. Results fold via unread(): OR the
booleans, max lastMsgAt over found rows, unreadDirectMessage = unread &&
roomType==dm, unreadMention = hasMention. Per-site RPC failures degrade
into unavailableSites. Grouping by siteId counts each thread exactly once.

- pkg/subject: UserThreadUnread/Pattern, ThreadRoomInfoBatch/Subscribe.
- pkg/model: ThreadUnreadRequest/Response, ThreadRoomInfoBatchRequest/
  Response, ThreadRoomInfo, ThreadSubRef.
- room-service: GetThreadRoomInfos store method (two projected finds,
  no $lookup) + threadRoomInfoBatch handler.
- user-service: roomclient.GetThreadRoomInfoBatch,
  mongorepo.ThreadSubscriptionRepo.ListByAccount, RoomClient/
  ThreadSubscriptionRepository interfaces, GetThreadUnread aggregator.
- docs/client-api.md: documents the new thread.unread RPC.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BocBvJn2N9YYMmciA5EWJX

Author: claude

docs/client-api.md

@@ -3776,6 +3776,7 @@ Additional legacy fields may be present, mirroring the `GET /api/v3/users` respo
 | `chat.user.{account}.request.user.{siteID}.subscription.setAppSubscription` | [`subscription.setAppSubscription`](#subscriptionsetappsubscription) |
 | `chat.user.{account}.request.user.{siteID}.apps.list` | [`apps.list`](#appslist) |
 | `chat.user.{account}.request.user.{siteID}.thread.list` | [List User Threads](#list-user-threads) |
+| `chat.user.{account}.request.user.{siteID}.thread.unread` | [Get Thread Unread](#get-thread-unread) |
 
 #### status.getByName
 
@@ -4532,6 +4533,58 @@ See [Error envelope](#6-error-envelope-reference). A malformed `cursor` returns
 
 ---
 
+#### Get Thread Unread
+
+**Subject:** `chat.user.{account}.request.user.{siteID}.thread.unread`
+**Reply subject:** auto-generated `_INBOX.>` (NATS request/reply)
+
+- `{siteID}` is the **caller's own home site** — the site that holds the user's federated subscriptions and runs the aggregator.
+
+Returns the aggregated unread status of the user's thread subscriptions across every site the user participates in. `user-service` determines those sites from the user's local thread-subscription replica, queries each owning site's `room-service` for the threads' latest activity, and merges the results into one response. Sites that fail to respond are listed in `unavailableSites` rather than failing the request.
+
+##### Request body
+
+Empty object.
+
+```json
+{}
+```
+
+##### Success response
+
+| Field | Type | Notes |
+|---|---|---|
+| `unread` | boolean | Any thread has activity newer than the user's last-seen. |
+| `unreadDirectMessage` | boolean | `unread` restricted to DM-room threads. |
+| `unreadMention` | boolean | The user is @-mentioned in any unread-tracked thread. |
+| `lastMessageAt` | number? | Optional. UnixMilli of the newest thread activity across sites; omitted when none. |
+| `unavailableSites` | string[]? | Optional. Sites whose per-site lookup failed; omitted when all responded. |
+
+```json
+{
+  "unread": true,
+  "unreadDirectMessage": false,
+  "unreadMention": true,
+  "lastMessageAt": 1717000000000
+}
+```
+
+##### Error response
+
+| Condition | `code` | Notes |
+|-----------|--------|-------|
+| Internal failure | `internal` | Local thread-subscription read failed. Per-site RPC failures degrade into `unavailableSites` rather than erroring. |
+
+##### Triggered events — success path
+
+`None — reply only.`
+
+##### Triggered events — error path
+
+`None — error returned only via the reply subject.`
+
+---
+
 ## 4. Message Send
 
 ### Send Message