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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.

Add group_id and validity_days fields to CreateAndRedeemCodeRequest,
enabling subscription-type redemption codes to be created and redeemed
in a single API call.

- Type defaults to "balance" when omitted for backward compatibility
- Subscription type requires group_id (non-nil) and validity_days (>0)
- Existing balance/concurrency callers are unaffected