Apply the same FOR UPDATE locking pattern across 8 providers:
- RioPay: added FOR UPDATE lock (had none at all)
- CryptoBot: moved lock before status check, removed redundant lock
- WATA: moved lock before is_paid commit, removed redundant lock
- Freekassa: moved lock before is_paid commit, removed redundant lock
- KassaAI: moved lock before is_paid commit, removed redundant lock
- MulenPay: moved lock before is_paid commit, removed redundant lock
- Pal24: moved lock before is_paid commit, removed redundant lock
- SeverPay: moved lock before is_paid check, removed redundant lock
Pattern applied to all: acquire FOR UPDATE with populate_existing=True
immediately after finding the payment, replace intermediate commits with
inline assignments + flush(), re-check is_paid from locked row.
Fringg
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