Stacked on #26657 (the persisted whole-chat summary backend). Base
branch is `chat-summary-62j9`; review/merge that first.
Adds a reusable `ChatSummary` component.
The summary text is the persisted whole-chat summary (`chat.summary`)
introduced by #26657. It is generated asynchronously and may be `null`
until the first summary is produced, in which case the popover renders a
muted empty state. Live updates arrive via that PR's
`chat_summary_change` watch event, which is already merged into the chat
caches.
Cost is served by a new per-chat endpoint, `GET
/api/experimental/chats/{chat}/cost`, which rolls up assistant-message
cost across a chat's root and child (subagent) chats and is authorized
like the other `{chat}` routes (read on the chat, 404 otherwise).
Visual and interaction coverage lives in `ChatSummary.stories.tsx` and
`ChatSummaryPopover.stories.tsx` (including populated-summary,
empty-state, and cost-loading cases).
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Co-authored-by: Cursor <cursoragent@cursor.com>
Adds a persisted whole-chat summary that backs the chat summary popover.
A new nullable `chats.summary` column is populated in the background
after a successful root-chat turn and pushed to clients via a new
`chat_summary_change` watch event (distinct from `summary_change`, which
is bound to `last_turn_summary`), so the popover reads `chat.summary`
straight off the loaded `Chat` with no extra query.
This is the data source for the popover and per-chat cost UI built in
#26649; the popover can consume `chat.summary` once this lands (the
field is nullable, so merge order does not matter).
## How it works
- **Generation** runs in the existing successful-turn finalize hook,
detached from the request so the user's turn is never blocked. A cadence
gate generates the first summary after one completed turn, then
regenerates every three turns, using the `chats.summary_generated_at`
freshness marker. Generation reads compaction-aware history, renders it
to a bounded plain-text transcript (short transcripts are skipped), and
asks for a 1-3 sentence summary via structured output. Failures never
clear an existing summary.
- **Staleness** is guarded by `history_version` (mirroring
`last_turn_summary`), so a background write racing a newer turn loses
while worker lifecycle transitions cannot reject a fresh write.
- **Model selection** uses the chat's configured model.
## Deferred to follow-ups
- **Cost accounting**: the `chat_messages.cost_source` discriminator and
summary/title usage recording were removed from this PR so summary
persistence is not blocked by hidden accounting rows advancing
`history_version`. Title usage recording stays on main's
`InsertChatMessages` path.
- **Model override**: deployment-wide summary generation model selection
is split into #26803; the base feature always uses the chat model.
## Notes
- Migration `000540` adds `chats.summary` and
`chats.summary_generated_at`, and recreates `chats_expanded` to expose
the new columns.
- Root chats only; shared viewers pick up the summary on their next
refetch (live watch events are owner-only).
Refs #26649
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Co-authored-by: Cursor <cursoragent@cursor.com>
