Rename session_id and user_session identifiers to user_id

[joebew42]

The value that flows through the backend as session_id, session_ids, or user_session is really the user's identifier — the username established in CreateUser. Naming it session_id is misleading, and several use cases (SubscribeToUserSession, JoinChatRoom, SendMessageToChatRoom) already call this parameter user_id. This change makes the naming consistent end-to-end so the identifier states what it actually is.

This PR

Renames the identifier variables, parameters, struct fields, and helper function names that denote the user identifier — session_id → user_id, session_ids → user_ids, and user_session (where it holds the id value) → user_id — across the controller, supervisors, GenServers, use cases, and their tests. This is a purely internal, behaviour-preserving refactor. The session-process abstraction names (ExChat.UserSession(s), UserSessionRegistry, SubscribeToUserSession) and the repository function find_user_session_by/1 with its message atoms are intentionally left untouched, since they name the session actor rather than the identifier. The WebSocket wire protocol carries the identifier only in the unchanged from field, so no frontend changes are required.

Closes #12