🐛 Fix quiz total marks calculation when question order is random

[b-l-i-n-d]

• Replaces multiple PHP usort/array_slice loops with a single SQL query in Quiz::get_quiz_total_marks()
• Returns 0 immediately when random order is active and a question limit creates a true random subset (COUNT > max_questions)
• Returns the full SUM for random order when all questions are shown (no limit, or limit ≥ pool size)
• Deterministic orders (asc, desc, sorting) use a single subquery with an allowlisted ORDER BY clause and LIMIT
• No-limit case (max_questions <= 0) uses a plain SUM query with no subquery or LIMIT
• ORDER BY clause is built from a fixed PHP allowlist — no user input reaches the SQL string