Redeem a promotion code against a charge or invoice

Apply a promotion code to a gross amount. Returns the discount amount, the new net amount, and the redemption record. Increments the redeem counters on both the coupon and the promotion code atomically.

Validates: code exists, coupon active, within validity window, not at redemption cap (global, per-code, per-customer), and (if restricted) matches the redeeming customer.