fix: CryptoBot guest payment — remove is_paid @property write, use correct status

- Remove `locked.is_paid = True` — is_paid is a read-only @property computed from status
- Change status from 'completed' to 'paid' (matching is_paid property check)
- Use db.commit() instead of db.flush() for guest payment persistence

app/services/payment/cryptobot.py

@@ -228,10 +228,9 @@ class CryptoBotPaymentMixin:
                     skip_amount_check=True,  # USD->RUB conversion introduces imprecision
                 )
                 if guest_result is not None:
-                    locked.status = 'completed'
-                    locked.is_paid = True
+                    locked.status = 'paid'
                     locked.paid_at = datetime.now(UTC)
-                    await db.flush()
+                    await db.commit()
                     return True
 
             if not updated_payment.transaction_id: