Render DocumentTemplate as PDF

Renders a document template with the provided data to PDF format.

  The template receives the TemplateVariables data structure from the request.
  All properties are optional and accessible via Liquid syntax (e.g., {{ PrimaryContact.FirstName }})

  Example request:
  POST /api/tenants/acme/document-templates/engagement-letter-standard/render
  {
    'designThemeCode': 'professional-blue',  // optional
    'data': {
      'title': 'Engagement Letter',
      'client': {
        'primaryContact': { 'firstName': 'John', 'lastName': 'Doe' },
        'partner': { 'name': 'Jane Smith', 'email': 'jane@example.com' }
      },
      'practice': {
        'address': '123 Main St',
        'email': 'info@practice.com',
        'telephone': '555-1234'
      }
    }
  }