Add Audit Logging for Submission Deletions

[TryOmar]

Issue: Add Audit Logging for Submission Deletions

Description

We need a logging system to track all submission deletions. This helps detect users who upload temporarily just to unlock solutions.

Requirements

• Create submission_logs table to track deletions
• Log:
  • submission_id
  • deleted_by_user_id
  • reason (e.g., "user delete", "admin delete")
  • timestamp
• Distinguish between user deletions and admin deletions
• Logs should be visible only to admins
• This provides transparency and protects against misuse