feat(grading): validate tutor review scores and feedback (BA-043)

[ranjeetprabhatedu-dotcom]

Summary

This PR addresses issue #956 (BA-043) by enforcing score bounds (0–100 integer) and required feedback/reason validation on tutor reviews with comprehensive boundary test coverage.

• Changes:
  • BackendAcademy/src/grading/dto/tutor-review.dto.ts: Added feedback field with class-validator decorators, validated score bounds (0 to 100), and enforced required feedback/reason rules for override decisions.
  • BackendAcademy/src/grading/grading.controller.ts: Validated integer score bounds (0 <= score <= 100) and enforced non-empty feedback/reason requirements for override decisions before delegating to GradingService.
  • BackendAcademy/src/grading/grading.service.spec.ts: Added unit tests verifying boundary scores (0, 100), rejection of negative, >100, and fractional scores, missing feedback/reason rejection, and feedback field equivalence.
• Verification:
  • All 21 tests in grading.service.spec.ts pass cleanly (100% Green).
  • Scope strictly restricted to the 3 allowlisted files.

Closes #956