feat: Add full text search over chat messages (#27126)

Closes CODAGT-721
Closes CODAGT-722
Closes CODAGT-723
Closes CODAGT-724
Closes CODAGT-725

This PR adds the database and API pieces necessary to support full-text
chat message search.

- Adds required chat schema for full-text search
- Adds dbpurge job to populate search_tsv in the background
- Adds `search` parameter to GetChats query
- Adds `search` filter to `searchquery.Chats`
- Wires chat search filter into chats API

> Implemented by Coder Agents, reviewed and tested by a human.

coderd/database/modelqueries.go

@@ -786,6 +786,7 @@ func (q *sqlQuerier) GetAuthorizedChats(ctx context.Context, arg GetChatsParams,
 		arg.PrNumber,
 		arg.RepoQuery,
 		arg.PrTitleQuery,
+		arg.Search,
 		arg.OffsetOpt,
 		arg.LimitOpt,
 	)