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/models.go

@@ -5123,6 +5123,8 @@ type ChatMessage struct {
 	Revision            int64                 `db:"revision" json:"revision"`
 	// Stores the selected effort for the turn triggered by this message.
 	ReasoningEffort NullChatReasoningEffort `db:"reasoning_effort" json:"reasoning_effort"`
+	// Used for full text search. NULL initially, populated async via background job.
+	SearchTsv interface{} `db:"search_tsv" json:"search_tsv"`
 }
 
 type ChatModelConfig struct {