Add transaction support to ClanShop module

[jkrizsan]

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[CapoMK25]

ClanShop module already uses MongoDB transactions via manual session handling.

Found on the code on clanShop.service.ts. ClanShopService already wraps multi-step operations (buyItem, checkVotingOnExpire) in MongoDB transactions using startSession/startTransaction. No further changes required (at least I think so).

[CapoMK25]

After a careful check I can see that transaction support is indeed implemented but only to an extent. Reopening this and finishing the job here for good.

[CapoMK25]

PR done yesterday, waiting for review and eventual merge.