feat(backend): express-validator for update-profile + package.json fix

[Mrgig7]

Summary

• Add controller-level validators: username (min 3), bio (<=150, optional), social_links.website (valid URL, optional)
• Return first validation error as 403 with { error: message } for frontend compatibility
• Preserve platform-specific hostname checks for other social links
• Fix malformed backend/package.json and install express-validator

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[Avdhesh-Varshney]

Closed because for any contribution, first raise the issue and wait for assign. Then finally raise the PR.

This is PR is raised without any discussion and prior info.