fix: always require KYC Level 30 for RealUnit purchases

[TaprootFreak]

Summary

• Remove the amount-based KYC Level 50 requirement for REALU buys in PUT /realunit/buy
• All RealUnit purchases now consistently require only KYC Level 30, regardless of the purchase amount
• Previously, amounts exceeding Config.tradingLimits.monthlyDefaultWoKyc incorrectly required KYC Level 50

Test plan

• Verify REALU purchase with amount below 1000 CHF still works with KYC Level 30
• Verify REALU purchase with amount above 1000 CHF now works with KYC Level 30 (previously required Level 50)
• Verify users below KYC Level 30 still get KycLevelRequiredException