fix: prevent players from joining multiple games at once

[sudojunior]

Players should be able to request to join other games, but it should not accept them in if they are already accepted into another. Running this as an 'active search' operation, would increase unneccesary requests to the API (as mentioned in #20 with ratelimits). Instead, while rather unoptimistic, it should be achievable to reject a join request and/or an acceptance if the player has already joined another game - therefore reducing excessive requests to multiple channels and making resources available elsewhere.

[sudojunior]

Intended join request flow (mermaid diagram)

stateDiagram-v2
    [*] --> Request

    state Request <<fork>>
    Request --> Public: game is public
    Request --> Private: game is private

    Accept --> Join
    Public --> Join
    Private --> Defer

    Defer --> Accept: host accepts request
    Defer --> Decline: host declines request

    Join --> Decline: player has joined another game

    Decline --> [*]
    Join --> [*]

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