In-memory session service backed by ETS tables.

Uses a GenServer to own three ETS tables:

• sessions — stores session data keyed by {app_name, user_id, session_id}
• app_state — app-level state keyed by app_name
• user_state — user-level state keyed by {app_name, user_id}

Reads go directly to ETS (with read_concurrency: true). Writes are serialized through GenServer calls.