Builds on Chat V2 (#5076). Adds a per-conversation pin so a user can
keep important chats at the top of the IM-style thread list, above the
activity-sorted rest.
Backend:
- migration 148: chat_session.pinned_at (nullable) + partial index; the
timestamp doubles as the pinned-group sort key and the boolean flag.
- list queries order pinned-first, then by most-recent activity.
- SetChatSessionPinned query + PATCH /api/chat/sessions/{id}/pin handler;
pinning never bumps updated_at, so an unpinned chat won't jump the list.
- ChatSessionResponse.pinned + chat:session_updated carries the new state.
Frontend:
- ChatSession.pinned; setChatSessionPinned API + useSetChatSessionPinned
with optimistic re-sort; shared sortChatSessions comparator.
- thread list: pin indicator on pinned rows + pin/unpin hover action;
list sorted pinned-first so it stays ordered after cache patches.
- realtime patch re-sorts on pin change; en/ja/ko/zh-Hans strings.
Tests: SetChatSessionPinned handler test, sortChatSessions unit tests.
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