finishing domain model structs

[adarshm11]

Finish Domain Model Structs

3 of 8 models exist in internal/core/models/models.go (AssessmentTemplate, Question, TestCase). The remaining 5 need to be added.

Already done

• AssessmentTemplate — id, title, description, durationMinutes, timestamps
• Question — id, assessmentTemplateId, type, prompt, options, correctAnswer, score, language
• TestCase — id, questionId, input, expectedOutput, isHidden, score

Remaining work

Add the following structs to internal/core/models/models.go:

• AssessmentInvite — id (int64), assessment_template_id (int64), candidate_email (string), invite_token (string), expires_at (time.Time), status (string), created_at, updated_at
• AssessmentAttempt — id (int64), invite_id (int64), started_at (*time.Time), completed_at (*time.Time), status (string), total_score (*int), created_at, updated_at
• Answer — id (int64), attempt_id (int64), question_id (int64), response (json.RawMessage), score_awarded (*int), created_at, updated_at
• Submission — id (int64), attempt_id (int64), question_id (int64), language (string), source_code (string), status (string), stdout (*string), stderr (*string), execution_ms (*int), memory_bytes (*int64), score_awarded (*int), created_at, updated_at
• SubmissionResult — id (int64), submission_id (int64), test_case_id (int64), passed (bool), actual_output (string), execution_ms (*int), created_at

All structs need json:"..." and db:"..." tags. Use pointer types for nullable fields consistent with existing models.