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/dump.sql

@@ -1215,7 +1215,8 @@ CREATE TABLE chat_messages (
     cache_creation_tokens bigint,
     cache_read_tokens bigint,
     context_limit bigint,
-    compressed boolean DEFAULT false NOT NULL
+    compressed boolean DEFAULT false NOT NULL,
+    created_by uuid
 );
 
 CREATE SEQUENCE chat_messages_id_seq