Generate a pre-signed S3 URL for uploading content reports. Requires authentication.

Use Case: Users can report inappropriate content or issues. This endpoint provides a secure upload URL that expires after 60 seconds.

Process:

1. Call this endpoint to get a signed URL
2. PUT your report JSON directly to the signed URL
3. Report is stored privately for moderation review

Security: The signed URL is temporary and scoped to the authenticated user.