## Summary
This PR removes two redundant chat rereads in `chatd`.
### Archive / unarchive
- `archiveChat` and `unarchiveChat` already come through
`httpmw.ChatParam`, so the handlers already have the `database.Chat`
row.
- Pass that row into `chatd.ArchiveChat` / `chatd.UnarchiveChat` instead
of rereading by ID before publishing the sidebar events.
### End-of-turn cleanup
- `processChat` no longer calls `GetChatByID` after the cleanup
transaction just to refresh the chat snapshot.
- Title generation already persists the generated title and emits its
own `title_change` event.
- To preserve best-effort title freshness for the cleanup path, the
async title-generation goroutine stores the generated title in per-turn
shared state and cleanup overlays it if available before publishing the
`status_change` event and dispatching push notifications.
## Why
- removes one DB read from archive / unarchive requests
- removes one DB read from completed turns, which is the larger hot-path
win
- keeps the existing pubsub/event contract intact instead of broadening
this into a larger event-model redesign
## Notes
- `title_change` remains the authoritative title update for clients
- cleanup does not wait for title generation; it uses the generated
title only when it is already available
Ethan
41bd7acf66