fleshing out repository layer

[adarshm11]

Build the Repository Layer

internal/core/repositories/repositories.go defines interfaces for AssessmentRepository, QuestionRepository, and TestCaseRepository — but there are zero concrete implementations. The remaining 5 entity interfaces are also missing.

Already done

• Repository interfaces for Assessment, Question, TestCase (interfaces only, in repositories.go)

Remaining work

1. Add missing interfaces to repositories.go

• InviteRepository — Create, GetByToken, ListByAssessmentID, UpdateStatus
• AttemptRepository — Create, GetByID, ListByAssessmentID, UpdateStatus, SetScore, SetCompletedAt
• AnswerRepository — Upsert, GetByAttemptID, SetScore
• SubmissionRepository — Create, GetByID, UpdateStatus, UpdateResult
• SubmissionResultRepository — CreateBatch, GetBySubmissionID

2. Implement all 8 concrete repos

Create one file per repo in internal/core/repositories/ (or a postgres/ sub-package). Each struct takes *sql.DB in its constructor.

• assessment_repo.go — implements AssessmentRepository (Create, List, GetByID)
• question_repo.go — implements QuestionRepository (Create, GetByID, ListByAssessmentID)
• test_case_repo.go — implements TestCaseRepository (CreateMany, ListByQuestionID)
• invite_repo.go — implements InviteRepository
• attempt_repo.go — implements AttemptRepository
• answer_repo.go — implements AnswerRepository (Upsert via INSERT ON CONFLICT)
• submission_repo.go — implements SubmissionRepository
• submission_result_repo.go — implements SubmissionResultRepository

Implementation notes

• Raw SQL with db.QueryRow / db.Query / db.Exec, no ORM
• JSONB columns use json.RawMessage
• Wrap "not found" vs real DB errors so callers can distinguish
• Tables already exist via internal/db/schema.go — match column names there

Note: this requires completion of #17