Checks the status of a voucher code without redeeming it. Returns the voucher's current state — status (ACTIVE/REVOKED), valid, redemption_count, redemptions_remaining, and reason when invalid.

Anonymous: campaign is sourced from the x-campaign-id header so the storefront can verify a code during the signup flow before the user has a session. Voucher state is per-campaign rather than per-user, so no caller identity is required.

The route is throttled at 20 RPS steady / 50 burst at the API Gateway stage level to deter brute-force enumeration of the voucher code space. Callers exceeding the cap receive HTTP 429.