1. PrepareRefund: block refund on provider instance lookup failure
instead of silently skipping permission check (medium severity)
2. UpdateProviderInstance: allow enabling refund_enabled and
allow_user_refund in the same request by checking req.RefundEnabled
value before falling back to DB read
3. ExecuteRefund: only revoke subscription on ErrAdjustWouldExpire,
abort on other errors (DB failure, not found) instead of
unconditionally revoking
c14d739360