Upload a supporting document for a group

Uploads a supporting document (license, certificate, insurance form, IRS document, etc.) for a group. The file is stored in cloud storage and its metadata is written into the group's supportingDocuments record list. No MIME-type or file-size restriction is currently enforced — any file is accepted and stored as application/octet-stream regardless of its actual type. Not idempotent — each call creates a new document entry even if the same file is uploaded again. The response contains only the uploaded fileName and a status message; it does not include the stored cloud storage path or URL.