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/migrations/000431_add_created_by_to_chat_messages.down.sql

@@ -0,0 +1 @@
+ALTER TABLE chat_messages DROP COLUMN created_by;

coderd/database/migrations/000431_add_created_by_to_chat_messages.up.sql

@@ -0,0 +1 @@
+ALTER TABLE chat_messages ADD COLUMN created_by uuid;