Allow redeem codes with negative values to enable refund scenarios:
- Balance: negative value deducts balance (clamped to 0, never negative)
- Concurrency: negative value reduces concurrency (clamped to 0)
- Subscription: negative validity_days reduces remaining days; if
remaining days <= 0, the subscription is canceled (set to expired)
All deductions generate standard redeem code records for audit trail.
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