feat: add luhn PIN generation and validation

[sahilthakurr24]

Summary

• Added Luhn check digit calculation for 5-digit payloads.
• Added 6-digit Luhn PIN generation.
• Added Luhn PIN validation.
• Added validation for invalid length and non-numeric PINs.

Testing

• npm test

Closes #580

[vercel]

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[github-actions]

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