Attach a new supporting document to a facility

Uploads and attaches a new supporting document to the specified facility. Use this to add a document for the first time; to update or remove an existing document, use PUT /facilities/{id}/supporting-documents instead (see that endpoint's description for its full-array-replace semantics). The file is uploaded as multipart/form-data and its bytes are read directly into GCS by this service — there is no pre-signed-URL pattern. There is no MIME-type allowlist; size is bounded only by the service's global request-body limits (currently 500MB body / 250MB per form attribute), not by a document-specific limit. fileType is a free-form optional string with no enforced enum or whitelist — callers may send any value. Returns HTTP 201 with only a message and the stored fileName — no document ID or file URL is returned by this endpoint.