feat(db): add created_by column to chat_messages table (#22940)

Adds a `created_by` column (nullable UUID) to the `chat_messages` table
to track which user created each message. Only user-sent messages
populate this field; assistant, tool, system, and summary messages leave
it null.

The column is threaded through the full stack: SQL migration, query
updates, generated Go/TypeScript types, db2sdk conversion, chatd
(including subagent paths), and API handlers. All API handlers that
insert user messages now pass the authenticated user's ID as
`created_by`.

No foreign key constraint was added, matching the existing pattern used
by `chat_model_configs.created_by`.

coderd/database/db2sdk/db2sdk.go

@@ -1059,9 +1059,14 @@ func ChatMessage(m database.ChatMessage) codersdk.ChatMessage {
 	if !m.ModelConfigID.Valid {
 		modelConfigID = nil
 	}
+	createdBy := &m.CreatedBy.UUID
+	if !m.CreatedBy.Valid {
+		createdBy = nil
+	}
 	msg := codersdk.ChatMessage{
 		ID:            m.ID,
 		ChatID:        m.ChatID,
+		CreatedBy:     createdBy,
 		ModelConfigID: modelConfigID,
 		CreatedAt:     m.CreatedAt,
 		Role:          m.Role,