Update the Schema section and related references throughout README.md to
accurately reflect the current 001_initial.sql migration:
- Rename 'users' table to 'sessions' with new columns: uuid, password_hash,
signing_key, away_message
- Add new 'clients' table (uuid, session_id FK, token, created_at, last_seen)
- Add topic_set_by and topic_set_at columns to 'channels' table
- Update channel_members FK from user_id to session_id
- Add params column to messages table
- Update client_queues FK from user_id to client_id
- Update Queue Architecture diagram labels and surrounding text
- Update In-Memory Broker description to use client_id terminology
- Update Multi-Client Model MVP note to reflect sessions/clients split
