Implement secure file upload with malware scanning

[Smartdevs17]

Description

File uploads (dispute evidence, KYC docs) are accepted without validation or scanning. Implement file type validation, size limits, and malware scanning.

Acceptance Criteria

• File type validation (MIME type, magic bytes)
• File size limits with configurable thresholds
• Malware scanning via ClamAV integration
• Secure file storage with access control
• Automatic quarantine of suspicious files
• File retention and scheduled cleanup

Technical Scope

• backend/src/middleware/
• backend/src/routes/
• infra/
• Edge: archive bombs (zip, tar), polyglot files, exif data leakage