fix: validate uploaded files using magic bytes

[charu2210]

Summary

Implemented file signature (magic byte) validation for uploaded files to prevent spoofed uploads that bypass extension or MIME-type checks.

Changes Made

• Added validateFileSignature middleware using file-type
• Validates actual file content instead of relying only on extensions/MIME types
• Rejects malformed or spoofed files with clear error messages
• Removes invalid uploaded files from disk before returning an error
• Integrated validation into the /answer-sheet upload route

Testing

• Renamed a .txt file to .pdf → correctly detected as invalid and rejected
• Verified that genuine PDF files are detected correctly through file signature analysis

Expected Outcome

• Fake files (e.g. .txt renamed to .pdf) are rejected
• Valid PDFs and supported images are accepted
• Upload validation is more secure and reliable

Fixes #63