feat: add request validation

[tusharshah21]

Summary

Implements a strict request validation framework for params, query, and body in the Talenttrust backend, with modular middleware, tests, and documentation as requested in issue #57.

Changes

• Added schema-based validator and reusable request validation middleware.
• Applied validation to contract routes for params, query, and body.
• Added unit and integration tests covering success and failure paths.
• Documented framework usage, validation behavior, and security assumptions/threat scenarios in README.md and docs/backend/request-validation-framework.md.

Verification

• Tests passed with coverage (npm test -- --coverage).
• TypeScript build passed (npm run build).
• Impacted modules meet the requested coverage target (>=95%).

Closes #57

[drips-wave]

@tusharshah21 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits