Adds the 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, 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 few turns, using a chats.summary_generated_at freshness marker. Generation reads compaction-aware history, renders it to a plain transcript, and asks for a 1-3 sentence summary via structured output.
• 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.
• Cost attribution adds a nullable chat_messages.cost_source discriminator. Summary spend is recorded as a hidden, soft-deleted accounting row tagged 'summary'; the existing manual-title accounting row is tagged 'title'. Ordinary turn spend stays NULL, so existing cost/usage queries are unchanged. The per-feature breakdown is left to the /cost endpoint (out of scope, feat: add chat summary popover and per-chat cost endpoint #26649).
• Model selection defaults to the chat's configured model, with a new deployment-wide summary_generation model override (mirroring title_generation); a configured-but-unusable override skips generation.

Notes

• Migration 000530 adds chats.summary, chats.summary_generated_at, and chat_messages.cost_source, 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