feat: validate quiz CSV uploads

[smitdodiya]

Problem:
Uploading a CSV with invalid data (e.g. an unrecognized question type) was silently accepted, showed a "Questions imported successfully" toast, and stored garbage. The bad data then crashed the quiz view with [GET] /api/v1/quizzes/{id}/questions: 500 Internal Server Error, because an invalid question type was silently coerced to 0, which no downstream lookup could resolve.

validation added:

• Question Type : Must be single answer or survey (now uses the shared CheckQuestionType helper instead of a silent map lookup). This is the fix for the 500.
• Question Text : Required (non-empty after trim).
• Options : At least 2 non-empty options required.
• Correct Answer : Required; each value must be numeric (now strconv.Atoi, no silent Sscanf); each must reference an option that actually exists (catches "answer = 7 but only 4 options").
• Correct Answer count : single answer → exactly one correct answer; survey → at least one.
• Points : Optional; if present must be a positive integer (rejects abc, 0, -5). Defaults to 1 when blank.
• Question Media / Options Media : Must be text, image, or code. Empty defaults to text (matching manual question creation). Values are trimmed + lowercased, so Image/CODE are accepted and stored normalized so the frontend renders them correctly.