Refactor & feat: Reintroduce assignments module and implement grade-based certificates

[AnvayYadav]

Description

This PR reintroduces the assignments-related tables and APIs while significantly enhancing the certificate system to support grade-based certificate assignment.

Key Changes:

Reintroduced:

• Entire assignments module including:
  • Assignment, AssignmentDto, AssignmentQueue entities
  • AssignmentSubmission and related DTOs
  • AssignmentsApiController and AssignmentSubmissionAPIController
  • AssignmentViewController and associated JPA repositories

My teammate removed this previously to reduce redundancy, but this action led to our last pull request breaking stuff.

Enhanced Certificate System:

• Introduced CertificateType enum with two tiers:

  • EXCELLENCE: Awarded for average scores ≥ 88%
  • COMPLETION: Awarded for average scores ≥ 70% but < 88%

• Extended UserCertificate entity to track:

  • Certificate type (EXCELLENCE or COMPLETION)
  • Sprint name
  • Average score
  • Date issued

• Implemented grade-based certificate assignment in UserCertificateController:

  • New /api/user-certificates/request endpoint accepts formative and summative assignments
  • Calculates average scores from Grade repository
  • Validates all assignments are completed before awarding
  • Supports certificate upgrades (COMPLETION → EXCELLENCE)
  • Prevents duplicate certificates for same sprint
  • Returns detailed response messages for different scenarios

• Updated CertificateRepository to remove quest dependencies

• Simplified CertificateController and CertificateViewController by removing quest-related logic

• Updated UI to remove quest selection from certificate creation form

[jm1021]

This looks a lot more contained. We have another pressing bug that we will test tomorrow and then try to get to this after stabilization.