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`.

codersdk/chats.go

@@ -49,6 +49,7 @@ type Chat struct {
 type ChatMessage struct {
 	ID            int64             `json:"id"`
 	ChatID        uuid.UUID         `json:"chat_id" format:"uuid"`
+	CreatedBy     *uuid.UUID        `json:"created_by,omitempty" format:"uuid"`
 	ModelConfigID *uuid.UUID        `json:"model_config_id,omitempty" format:"uuid"`
 	CreatedAt     time.Time         `json:"created_at" format:"date-time"`
 	Role          string            `json:"role"`