Kassa payment webhooks, IP-check, event log, idempotency, and emergency capture

▤ Summary

This update improves Kassa payment handling with webhook validation, capture workflows, and idempotency support. It adds IP-based webhook filtering and a payment event log so the backend can record each incoming event and track whether it was applied. The code also handles waiting-for-capture payments, confirms or captures payments from the backend, and syncs succeeded payments against local subscription records. The changes strengthen reliability for recurring billing and reduce duplicate or unsafe payment actions.

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