Upload proposal PDF

Uploads a PDF document for the proposal.

  The PDF should be sent as a base64-encoded string in the request body.

  Request body example:
  {
    "pdfContent": "JVBERi0xLjQKJeLj..."  // Base64-encoded PDF content
  }

  The uploaded PDF will be stored in Azure Blob Storage and associated with this proposal.
  The proposal's HasProposalPdf flag will be set to true upon successful upload.

  You can retrieve this PDF later using the GET /{code}/pdf/proposal endpoint.

  Note: Uploading a new PDF will replace any existing proposal PDF for this proposal.