[WIP] Address feedback on completion rate improvements

[Copilot]

• Analyze the feedback and current state of the code
• Review the changes already made to understand what's been addressed
• Verify that the implementation correctly tracks max position reached
• Ensure comments accurately reflect the behavior
• Update remaining comment at line 237 to use "최대 도달 위치" instead of "유니크 시청 시간"
• Perform final code review
• Reply to the review comment

Summary

Successfully addressed all feedback regarding the unique_watched_seconds field:

1. Implementation fixed: update_progress now uses max() to prevent value from decreasing on rewind
2. Documentation updated: All comments now consistently use "최대 도달 위치" (maximum position reached)
3. Model documentation clarified: Progress model clearly states the field's purpose and behavior

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