perf(chatd): remove redundant chat rereads (#23161)

## 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

coderd/chats.go

@@ -1386,7 +1386,7 @@ func (api *API) archiveChat(rw http.ResponseWriter, r *http.Request) {
 	// active subscribers. Fall back to direct DB for the
 	// simple archive flag — no streaming state is involved.
 	if api.chatDaemon != nil {
-		err = api.chatDaemon.ArchiveChat(ctx, chat.ID)
+		err = api.chatDaemon.ArchiveChat(ctx, chat)
 	} else {
 		err = api.Database.ArchiveChatByID(ctx, chat.ID)
 	}
@@ -1417,7 +1417,7 @@ func (api *API) unarchiveChat(rw http.ResponseWriter, r *http.Request) {
 	// active subscribers. Fall back to direct DB for the
 	// simple unarchive flag — no streaming state is involved.
 	if api.chatDaemon != nil {
-		err = api.chatDaemon.UnarchiveChat(ctx, chat.ID)
+		err = api.chatDaemon.UnarchiveChat(ctx, chat)
 	} else {
 		err = api.Database.UnarchiveChatByID(ctx, chat.ID)
 	}