fix(lobby): Make the lobby server thread-safe

[tintinhamans]

Lobby state was changed from HTTP handlers, websocket handlers and timers without consistent locking. That allowed duplicate lobby IDs, slot corruption during host migration, and leaves that finished after the next create or join.

• Lobby IDs and anti-cheat probe counters are atomic.
• Every change to a lobby goes through one per-lobby gate, including host migration, CloseOpenSlots and kicks. The kick's removal runs after the gate is released.
• LeaveAnyLobby is awaited on create, join and delete.
• slot_state and lobby update fields are validated (400 on bad values).
• The session sweep re-checks before destroying a session, so a reconnect can't lose its lobby. The reconnect grace period is configurable.