As survey owner I want to make files uploaded to certain fields publicly accessible

We need to implement a field-level configuration option for files uploaded through our survey forms. Currently, all uploaded files are stored in MinIO and require authentication to access, which limits our ability to display these files on public-facing pages.

## Background
The current implementation stores all uploaded files (including PNG images) in a MinIO bucket with restricted access. For some use cases, we need to make specific uploaded files publicly accessible:

- Event program images that need to be displayed on public web pages
- Location maps for attendees
- Partner/sponsor logos that need to appear on the public site
- Other publicly shareable assets uploaded via forms

## Requirements

1. Add a configuration toggle at the field level in the form builder that allows marking uploaded files as "public"
2. For files marked as public:
- Either move them to a public MinIO bucket
- Or implement automatic transfer to WordPress media library
- Or generate presigned URLs with appropriate cache configuration

3. Maintain existing security for sensitive uploads (e.g., artist portfolios) by defaulting to private

## Success Criteria
- Form admins can configure file upload fields as public or private
- Public files are accessible via direct URLs without authentication
- Private files maintain their current security requirements
- Solution works reliably with our existing form submission flow

## Notes
The solution approach is flexible - whether using MinIO public buckets, WordPress media library integration, or presigned URLs with caching. The key requirement is enabling public access to specific files while maintaining security for sensitive uploads.

[Slack discussion (Tracon)](https://traconfi.slack.com/archives/C066E0DC5NX/p1743593303544839?thread_ts=1743511829.949609&cid=C066E0DC5NX)

AI disclaimer: The issue description has been generated from an excerpt of the linked Slack thread using Claude and manually edited further.