Issue

Build unsigned XDR to issue a VC via the Issuance contract.

POST /tx/prepare/issue

• Body: { owner: string; vcId: string; vcData: string; vaultContractId?: string }

• Returns: { unsignedXdr: string }

• Validation errors: owner_required, vcId_required, vcData_required, bad_request (simulation)

• Source: api/testnet/src/controllers/txPrepareIssueController.ts:5-27