Upload a KYC document

Upload ONE document and get back its provider document id, to reference from POST /v1/phone-numbers/kyc via documents[].documentId. Send the RAW file bytes as the request body (not base64); put the filename in the X-Filename header. Uploading documents one-per-request keeps each request under the ~4.5MB body limit. The document streams straight to the number provider and is not stored by Zernio.