Pass user_id to the server when connecting.

Is there a way on the client side to transfer user_id (user ID registered on the site) and referrer (address of the page from which the connection was made) to the server, so that each connection can be checked for user_id and referrer?

Start by reading the client-side connection flow and the server-side connection or handshake entry point to determine how connection metadata is currently transferred. Clarify the expected format and validation for user_id and referrer, then add coverage showing that both values reach the server and can be checked.

Written by the indexing model from the issue text.