Handle case where user joins a group when it's in read-only state (and becomes writable later on)

[LeeviHalme]

When a user joins a group chat when it's read-only, the bot skips the challenge as intended.

• If the group chat is later changed back to writable, that already-joined user is not challenged retroactively.
• This is current behavior introduced in PR Fix: Skip bot challenge in read-only groups #6 and may be acceptable, but it is an unresolved decision.

"Jätetään tämä järjestelmävastaavien murheeksi ✍️ ✍️ ✍️ "