fix: prevent multiple concurrent face verification requests

[VishnuVardhanCodes]

Description

This PR fixes the issue where multiple face recognition verification requests could be triggered simultaneously, leading to duplicate attendance submissions and unnecessary backend API calls.

Changes Made

• Added request locking to prevent concurrent verification requests
• Ignored additional verification attempts while a request is in progress
• Prevented duplicate attendance submissions
• Added server-side validation before attendance insertion
• Improved face recognition workflow reliability

Result

• Only one verification request is processed at a time
• Duplicate attendance records are prevented
• Reduced unnecessary backend API calls
• Improved security and system stability

Fixes #3907