feat(history-service): rooms.get — batch room last-message read (#393)

[chenjr0719]

Summary

Implements rooms.get (chat#393) — the room last-message endpoint (the customer's
/api/v1/rooms.get, used by mobile to render the last-message snippet in the room
list). A new history-service, per-site, account-scoped batch RPC that resolves each
requested room's latest message at read time (A2). Maintainer-approved approach (A2,
option E2) per the #393 design-lock.

What's included

• Subject chat.user.{account}.request.history.{siteID}.rooms.get — account+site from
  the subject, {roomIds[]} in the body. One batch per site; the caller groups a user's
  rooms by site and fans out (same shape as ThreadSubscriptionList).
• RoomsGet handler (history-service) → roomLastMessage: per room, the existing
  access check (checkAccessAndRoomTimes) then the latest message via the existing
  messages_by_room … DESC reads (GetMessagesBefore / GetMessagesBetweenDesc,
  limit 1) — access-window + history-floor aware (mirrors LoadHistory). No new repo method.
• A2 read-only: no denormalized write, no event, no edit/delete hooks, no
  walk-back — a soft-deleted latest message is returned as-is with deleted=true.
• Best-effort batch: bounded-concurrency per-room resolve; a room that's
  not-accessible / empty / errored is omitted, never failing the batch. Batch ≤ 100;
  content preview-trimmed to 256 runes.
• docs/client-api.md §3.2 "Get Rooms Last Message" (doc-ratchet).

#393 completion — web-sidebar integration

• Shared wire types moved to pkg/model (LastMessage, RoomsGetRequest,
  RoomsGetResponse) so user-service can consume them without importing
  history-service internals; history-service's own models package now aliases them
  (type LastMessage = model.LastMessage, etc. — no behavior change).
• SubscriptionRoom.LastMessage *model.LastMessage (pkg/model/subscription.go) —
  new optional field on the room-derived view returned by subscription.list.
• historyclient.Client.RoomsGet — new per-site RPC call mirroring the existing
  GetThreadList pattern (marshal request, NATS request/reply, errcode.Parse,
  unmarshal). Added to the HistoryClient interface; mocks regenerated.
• UserService.enrichWithLastMessage (user-service/service/subscriptions.go) —
  new enrichment pass in ListSubscriptions, run after enrichWithRoomInfo. Groups
  every subscription with a resolved Room by site (local and cross-site — unlike
  room-info enrichment, last-message isn't part of the local $lookup baseline), fans
  out one rooms.get call per site (bounded by the existing maxSiteFanout
  semaphore), chunking each site's roomIds at 100 to respect history-service's own
  batch cap. A degraded or absent site just leaves those rooms' lastMessage nil —
  it never fails the list.
• docs/client-api.md — added the lastMessage field to the SubscriptionRoom
  table + example, and reconciled the rooms.get RPC description to note
  subscription.list now calls it server-side (the RPC also remains directly callable).

Out of scope (follow-up)

• None — the user-service integration originally called out as follow-up is now
  included above.

Verification

• go build ./pkg/... ./history-service/... ./user-service/... — clean.
• go test ./pkg/... ./history-service/... ./user-service/... — green, including two
  new TestListSubscriptions_LocalBaselineRoom_NoKey assertions and a new
  TestListSubscriptions_LastMessage_SiteDegrades test (site RPC failure degrades
  gracefully, list still succeeds).
• NATS wire-layer e2e against the dev stack — to follow before marking ready.

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• Commit unit tests in branch feat/393-rooms-get

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