Add persistent rooms: server-side session state with configurable TTL

[fingolfin0]

Protocol changes

• JOIN_ACCEPTED payload now includes isNewRoom: boolean, signaling whether a saved session exists for the room.
• New client-to-server event SEED_FILE_STRUCTURE allows the first client in a new room to provide the initial file tree.
• When a saved session exists, the server sends SYNC_FILE_STRUCTURE to the joining client alongside JOIN_ACCEPTED.

How it works

1. First client joins a new room and sends SEED_FILE_STRUCTURE with its file tree; the server stores it.
2. Subsequent mutations are applied to the stored tree as they occur.
3. When a room becomes empty, a cleanup timer (default 60 seconds) starts. If a user rejoins before it fires, the timer is cancelled and the saved tree is sent to the new client.
4. If the timer expires, the session is discarded.

Note

This code has been tested in a live environment and works as expected.

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