View/download counters and download terms modal

[NishaSharma14]

Describe the issue
This feature request tracks the implementation of view/download counters and a download terms modal, as outlined in PR #1468.

Describe the solution you'd like
Implement the following features:

1. Download Security Improvements

   • Require valid UUID for downloads
   • Return 404 for missing, malformed, or unknown download UUIDs
   • Resolve S3 bucket from filesystem config instead of trusting request parameters

2. Interaction Tracking

   • Add view/download counters to projects and studies
   • Create InteractionTracker service that:
     • Deduplicates per session
     • Rolls up dataset and study interactions to owning project
     • Only counts public entities (excludes reviewer previews)
   • Record views on public pages
   • Record downloads via rate-limited POST /track/download/{identifier} beacon
   • Expose counters through project and study resources

3. Download Terms Modal

   • Display view and download counts on public project and sample pages
   • Gate downloads behind terms acceptance modal (DownloadTermsModal)
   • Use resources/markdown/download-terms.md for terms content
   • Fire download-tracking beacon on acceptance
   • Trigger download after terms acceptance