feat: add secure file browser, download, and delete management

[roninscript30]

🚀 Summary

This PR implements core file lifecycle features by adding:

• File download/retrieval UX using presigned GET URLs (Add file download/retrieval UX for presigned GET flow #7)
• File deletion flow including S3 object removal and database cleanup (Add file deletion endpoint and flow (S3 + history cleanup) #10)

These changes complete the end-to-end file management experience in the system.

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✅ Changes

🔽 File Download UX (#7)

• Added file listing UI with download action
• Integrated backend get-file-url endpoint
• Implemented presigned URL-based download flow
• Added loading and error handling states
• Handled URL expiry with retry mechanism

🗑️ File Deletion Flow (#10)

• Added DELETE endpoint: /api/upload/file/{file_id}
• Implemented S3 object deletion using AWS SDK
• Removed/soft-deleted file metadata from MongoDB
• Added delete button with confirmation UI
• Updated frontend state after deletion

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🧪 Testing

• ✅ Upload → Download → Works correctly
• ✅ Upload → Delete → Removed from UI and S3
• ✅ Refresh → Deleted files not present
• ✅ Error handling for failed API calls

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🎯 Impact

• Completes file lifecycle: Upload → View → Download → Delete
• Improves usability and demo readiness
• Aligns with core project requirements

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Closes #7
Closes #10