Create signed discount code

Creates a signed discount verification code (INSR-XXXXX) valid for 30 minutes. A code that carries a discount costs 1 regular credit from the API key that owns the store (the key that created it); a 0% result is free. A caller using another key is not charged; when the caller is the owner key, the caller's key pays. Stores run through a licensed platform such as Skye Meta are covered by that platform's license. If merchant has Stripe Connect, a coupon is auto-created. The order is fixed: the result is signed first, then the credit is taken and the code is stored together, and a coupon is created only after the code exists. A result that cannot be signed, or a code that cannot be stored, is a 503 in the standard error envelope with no credit used and no code created. A retry after a timeout on the caller's side is a new verification: if the first request completed, it used a credit and created a code, and the retry does both again. A coupon that cannot be created leaves a valid code with stripeCodeCreated false.