Add time-based debounce for quiz retry items to prevent consecutive related questions

[Copilot]

Problem

When users answer quiz questions incorrectly, the current behavior immediately presents:

1. The same question again for retry
2. A series of related questions consecutively after getting it correct

This makes the answers too obvious since related questions appear back-to-back, reducing the educational effectiveness.

Solution

Implemented a 5-minute time-based debounce for retry items that spreads out related questions while maintaining their priority:

Before:

❌ Question: What does 好 mean? → Wrong answer
⚡ IMMEDIATE: Same question again → Correct answer  
⚡ IMMEDIATE: Related question about 女 → Too obvious
⚡ IMMEDIATE: Related question about 子 → Too obvious

After:

❌ Question: What does 好 mean? → Wrong answer
⏱️ 5-MINUTE WAIT: Other questions appear first
✅ AFTER DELAY: Question about 好 returns (still prioritized)
📚 VARIED TIMING: Related questions spread out naturally

Technical Changes

• Added skillRetryDebounce constant: 5-minute delay before retry items become available
• Modified skillReviewQueue logic: Skills with Rating.Again are temporarily placed in the due queue with debounce end time, then moved to retry queue once the debounce period expires
• Maintained prioritization: Retry items still get highest priority once they become available again

Testing

• ✅ Added comprehensive test coverage for debounce behavior
• ✅ Updated existing retry logic tests to work with new timing requirements
• ✅ All 58 tests passing across skill review system
• ✅ Verified both scenarios: within debounce period and after debounce expiry

The implementation successfully prevents consecutive related questions while preserving the educational value of prioritizing incorrect answers for review.

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